Download Response of bankfull flood magnitudes to Holocene climate change

Response of bankfull flood magnitudes to Holocene climate change,
Uinta Mountains, northeastern Utah
Eric C. Carson†
Department of Geography and Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin
53706, USA
James C. Knox
Department of Geography, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
David M. Mickelson
Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
ABSTRACT
Long-term variations in Holocene flood
magnitude were quantified from the bankfull
dimensions of abandoned channels preserved
on floodplain surfaces in the northern Uinta
Mountains of northeastern Utah. Cross-sectional areas of abandoned channels were
reconstructed, and relationships derived
from the modern gage records were used to
estimate bankfull discharges from bankfull
cross-section areas. The results indicate systematic (nonrandom) variations of bankfull
floods in the northern Uinta Mountains.
Large floods, as much as 10%–15% greater
than modern, dominated from 8500 to 5000
calendar yr B.P., and again from 2800 to
1000 cal yr B.P. Small floods, as much as
15%–20% less than modern, characterize
the periods from 5000 to 2800 cal yr B.P., and
from 1000 cal yr B.P. to near present.
The middle and late Holocene record of
bankfull flood magnitude compares well
with independent evidence for climatic variation in the area. The early Holocene record
indicates that larger than modern bankfull
floods coincide with warmer than modern
mean annual temperature. We hypothesize
that an increased range of magnitude for seasonal solar radiation during the early Holocene favored the accumulation and rapid
melting of deep snowpacks in the high Uinta
Mountains, thus producing large floods
despite warmer mean annual temperatures.
The episode of smaller than modern bankfull floods between 5000 and 2800 cal yr B.P.
†
Current address: San Jacinto College Geology
Department, 5800 Uvalde Road, Houston, Texas
77049, USA; [email protected].
coincides with records of increased forest fire
frequency in the northern Uintas. Larger
than modern floods from 2800 to 1000 cal yr
B.P. coincide with a local decrease in forest
fire frequency and evidence for minor local
glacial readvances. The decrease in flood
magnitudes following 1000 cal yr B.P. corresponds to numerous local and regional
records of warming during the Medieval Climatic Anomaly.
Keywords: Holocene, floods, climate, subalpine,
Uinta Mountains, Medieval Climatic Anomaly.
INTRODUCTION
Durations of historic records of stream discharge are typically too short to provide a fully
adequate assessment of the relationship between
climate change and flood magnitudes. Moreover,
in many regions, the impacts of land use (e.g.,
urbanization, cultivation) and fluvial system
alterations (e.g., channelization, dam construction) hinder the ability to identify hydrogeomorphic responses to climate change. In the past two
decades there has been a growing recognition of
the limitations of flood-frequency analyses that
are based on assumptions of stationarity of the
mean and variance in time series, and a rise in
emphasis on paleoflood studies that address this
shortcoming of flood-frequency analyses (e.g.,
Knox, 1984, 1993, 1999, 2000; Baker et al.,
2001; Redmond et al., 2002; Baker, 2003).
Many of the investigations of Holocene
paleoflood variability in the United States have
focused on two regions, the Upper Mississippi
Valley and the southwestern U.S. Upper Mississippi Valley studies have emphasized both
high-frequency and low-frequency floods (e.g.,
Knox, 1985, 1993), whereas southwest studies
have largely focused on the relative importance
of North Pacific winter frontal systems and
Pacific tropical cyclones, and the lesser importance of monsoonal weather systems moving
north from Baja California, on flood genesis
(e.g., review by Ely, 1997). The southwest
research has been highlighted by work on the
Pecos River in Texas (Kochel and Baker, 1982,
1988), the Escalante River in Utah (Webb et al.,
1988), the Gila River in Arizona (Ely, 1992), the
Verde River in Arizona (Ely and Baker, 1985;
House et al., 2001), and regional syntheses of
the relationship between paleoflood behavior
and climate change (Ely et al., 1993; Ely, 1997).
Owing to the concerted efforts to understand the
significance of slackwater paleostage indicators
from large-magnitude floods (e.g., Costa, 1978;
Baker, 1987), minimal work in the southwest
has been focused on Holocene variations in
small-magnitude, high-frequency discharges.
The research presented here attempts to partly
fill this void.
Flood regimes for streams on the north flank
of the Uinta Mountains of northeastern Utah
are strongly influenced by the amount of winter
snowpack and the timing of spring snowmelt.
The magnitude of the peak annual flood on
streams in the Uintas is dependent on amount
of precipitation during the preceding winter,
amount of precipitation during the spring snowmelt season, and temperature during the spring
snowmelt season. Winter precipitation is significant as the source of the annual snowpack;
and spring precipitation and spring temperature
are significant as an additional source of snowpack and the determinant of the rate of spring
snowmelt. The historic stream gage records
for streams in the Uintas show that larger than
average peak annual floods tend to occur relatively late in the snowmelt season, resulting
GSA Bulletin; September/October 2007; v. 119; no. 9/10; p. 1066–1078; doi: 10.1130/B25916.1; 8 figures; 3 tables.
1066
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© 2007 Geological Society of America
Uinta Holocene bankfull flood
from unseasonably cool spring temperatures
allowing the winter’s snowpack to persist relatively late into the melt season and then be
released in a single pulse that is likely often
augmented by rain on snow events (Carson,
2003). In the Uintas, the annual nival (spring
snowmelt) flood dominates the peak-discharge
series, making this area ideal for reconstructing responses of high-frequency floods to longterm Holocene climatic variations. The general
objective of this research is to use floodplain
alluvial stratigraphy and relict channel morphology for quantitative reconstruction of the
magnitudes of former high-frequency floods
in the context of their response to Holocene
climatic changes. Knox (1985) developed a
methodology for evaluating the magnitudes of
former bankfull floods in the Upper Mississippi Valley based on the dimensions of relict
channels (cutoff oxbows). In humid climate
settings, the capacity of the bankfull channel
and the elevation of the associated floodplain
under construction are strongly related to the
magnitude of the most frequently occurring
(modal) flood discharge (Wolman and Leopold,
1957; Dury, 1974, 1976). Alluvial valley fills in
low-gradient meadows on the north flank of the
Uinta Mountains are favorable for the preservation of abandoned relict channels. Infilling of
the abandoned channels by natural alluvial processes has preserved the cross-section areas that
were likely attuned to the magnitudes of modal
floods at the time that each respective channel was abandoned. Reconstruction of the discharge responsible for the respective cross-section capacities for these relict channels provides
a record of the temporal variations in bankfull
flood magnitudes. Here we compare the results
of this reconstruction with independent records
of Holocene paleoclimate in the Uinta Mountains and surrounding regions to evaluate the
sensitivity of flood behavior to climate change
in this area.
SETTING
The main area of study is the north flank of
the Uinta Mountains in northeastern Utah. The
Uinta Mountains extend ~200 km eastward
from the Wasatch Mountains near Kamas, Utah,
into northwestern Colorado (Fig. 1). The range
is an east-west–trending, Laramide-age fore-
110o 30′
110o 15′
00
BUP
00
Uinta
Mountains
3500
0
350
0
35
0
40o 45′
30
study area
Salt
Lake
City
3500
3000
3000
CHM
BBM
E. F
k. B
la
30
ck’s
BSM
nr
y’s
WFB
3000
Fo
rk
EFB
HFN
E. Fk. Sm
it
Bear Rive
r
a
Bl
Fork
.
k
.F
W
rk
Fo
He
00
30
’s
ck
3000
00
W. Fk. Smith
’s
25
h’s Fork
Fork
o
41 00′’
110o 45′
land uplift cored by Proterozoic rocks of the
Uinta Mountain Group (Sears et al., 1982). The
streams in the Uinta Mountains occupy valleys
scoured by multiple late Wisconsin glaciations
(Atwood, 1909; Bradley, 1936; Richmond,
1965; Hansen, 1969; Laabs and Carson, 2005;
Munroe, 2005). Channel beds rest on bedrock
in headwater reaches, on boulders in confined
valley reaches, and on glacial outwash along
mainstem reaches of the streams (Carson,
2005; Schmidt et al., 2005). The sites selected
for this study are all located on stream reaches
in broad subalpine meadows underlain by glacial outwash or bedrock. Auger drilling at several of the study sites confirmed that the glacial
outwash underlying the floodplain sediments is
composed of cobbles and boulders to ~0.5 m
in diameter. The size of these sediments suggests that the streams have not incised into the
outwash during the Holocene, and have instead
predominantly meandered laterally across the
outwash surface. Despite the high elevation
of the study reaches (2650–2950 m above
sea level), channel gradients are commonly
<0.0005, much gentler than many streams in
many mountainous settings, but within the
5
150
5
0
km
Utah
10
C.I. = 500 m
Figure 1. Location map showing study area on north flank of the Uinta Mountains in northeastern Utah. Shaded boxes indicate field sites
for coring relict channels. Field sites: BBM—Broadbent Meadow; BUP—Buck Pasture; BSM—Boy Scout Meadow; CHM—Christmas
Meadows; EFB—East Fork Black’s Fork; HFN—Henry’s Fork Narrows; WFB—West Fork Black’s Fork.
Geological Society of America Bulletin, September/October 2007
1067
Carson et al.
range of values associated with stepped glacial
valleys, moraine-dammed valleys, and lowelevation alluvial streams. Bedload sediments
that are moved through these reaches at or near
bankfull stage consist of sand and gravel to
small boulders derived primarily from resistant
quartzites of the Uinta Mountain Group. The
suspended-sediment loads largely consist of
silt and clay derived from the Red Pine Shale
formation of the Uinta Mountain Group.
The study site is well located to investigate the response of flood hydrology to climate change. Uinta Mountain precipitation is
derived in large part from two distinct meteorological systems that vary seasonally. Winter
precipitation and resultant snow accumulation, the source of spring runoff, are generally
associated with North Pacific frontal storms
associated with eastward-moving low-pressure
systems (Pyke, 1972; Hansen and Schwartz,
1981). Summer precipitation is derived from
eastern North Pacific extratropical cyclones
that are steered into Utah by either a deep lowpressure trough or a cutoff low-pressure system
in the upper-level westerly circulation along
the west coast of the United States (Mitchell,
1976; Maddox et al., 1980). Summer convective
storms can provide intense, localized rainfall.
The stream basins in this study are sufficiently
high in elevation that snowmelt runoff—and
therefore winter snowpack accumulation and
the timing and rate of spring snowmelt—is the
dominant flood-producing mechanism (Jarrett, 1990). We compiled 25 local stream gage
records, with an average length of record of
57.0 yr, for this study. The longest of these gage
records are Duchesne River at Myton, Utah, and
Weber River at Oakley, Utah, with uninterrupted
peak annual discharge records dating from
1901 and 1905, respectively. From these data,
the recorded peak floods have been associated
with the spring snowmelt in >95% of gage years
on Uinta mountain streams. Because the peak
annual floods in the northern Uinta Mountains
are almost exclusively derived from the spring
snowmelt, these stream systems are sensitive
to variations in the amount of snowpack accumulated during the winter months, and to the
timing of snowmelt during the spring and early
summer months.
METHODS
We sought to identify long-term variations in
the magnitude of the bankfull-stage discharge
for streams in the northern Uinta Mountains
in response to past climate change. Therefore,
accurate recognition of bankfull stage and discharge are critical to this study. Data from U.S.
Geological Survey stream gages in the Uinta
1068
Mountains suggest that the statistically most
probable (modal) flood has a recurrence interval of between 1.8 and 2.2 yr. For this study,
the bankfull-stage discharge was defined as
having a 2.0 yr recurrence interval; this was
employed as the reference for comparing modern flood discharges against flood discharge
estimates for ancient channels, and provides a
link between discharges of a known recurrence
interval with bankfull conditions that can be
identified in the field.
In this study we combined recognition of
the upper limit of sand-size particles with the
elevation of the floodplain under construction to
define the bankfull stage. In the northern Uinta
Mountains study area, the floodplain under
construction is composed of a fining-upward
sequence of lateral-accretion (point-bar) sediments. While there is significant local variation
in the grain size of point-bar sediments, ranging from coarse pebbles to medium sand, these
sediments contrast markedly with the silt- and
clay-dominated vertical-accretion (overbank)
sediments on the floodplain surface. This pronounced sedimentological break between the
sand-size particles and finer materials corresponds with the tops of the modern point bars.
Employing this sedimentological definition
allows for consistent determination of bankfull
stage in both modern channels that are surveyed
at the surface and ancient channels that are preserved in floodplain and terrace alluvial deposits. This method therefore combines the close
correspondence between the statistically defined
2.0 yr recurrence interval flood and bankfull
conditions with the sedimentological definition
of bankfull conditions that can be identified in
both modern and ancient channels.
Individual relict abandoned channels were
used to estimate the magnitude of the channel-forming flood at the time they were cut off.
Abandoned channels were cored at short intervals to measure the bankfull cross-section area,
as identified by geomorphic evidence. For each
cored channel, the bankfull cross-section area
was employed to estimate bankfull discharge
using a regression equation derived from modern hydrologic data from the northern Uintas.
Where possible, the age of abandonment for
each of the cored channels was established
directly by radiocarbon dating basal of organic
channel-fill sediment. For channels without
sufficient material for radiocarbon dating, age
was estimated using the thickness of verticalaccretion sediment on the associated point bar.
To remove the effect of varying drainage areas
for the paleochannels, the paleodischarge for
each abandoned channel was then compared
to the modern bankfull discharge of the stream
immediately adjacent to it.
Coring Abandoned Channels
Abandoned alluvial channels are preserved
within the floodplain sediments of seven meadows across the northern Uinta Mountains (Fig. 1).
In order to accurately reconstruct bankfull paleochannel dimensions, coring transects must be
oriented perpendicular to paleoflow direction.
Therefore, abandoned channels were selected
only if both the point bars and cutbanks of the
abandoned channels were clearly evident from
surface morphology. Most relict channels have
been filled and have only 5–10 cm of topographic relief on the floodplain, but are often
visible in aerial photography (Fig. 2) and during field inspection. From the 7 meadows used
in this study, a total of 30 relict channels were
identified for coring: 2 channels in Broadbent
Meadow, 2 in Boy Scout Meadow, 1 in Buck
Pasture, 4 in Christmas Meadows, 3 in East Fork
Black’s Fork, 3 in Henry’s Fork Narrows, and 15
in West Fork Black’s Fork (Fig. 1).
Transects across 26 channels were sampled
with an Oakfield 2-cm-diameter soil probe; cores
were spaced at 0.5–1.0 m intervals. Sediment
was described as it was collected and discarded.
Samples for radiocarbon dating were collected
by taking multiple samples from organic-rich
horizons at identified depths. Transects across
4 additional relict channels were sampled at
a 1.0–1.5 m interval with a modified 7.5-cmdiameter Vibracoring system (Carson, 2003).
Additional Oakfield cores were collected at
increased spacing as far as 20 m beyond channel
boundaries to verify that no significant channel
incision or aggradation had occurred. (For full
descriptions of all cores, see Carson [2003].) In
the West Fork Black’s Fork meadow, two Vibracore transects were collected across the meadow
and further confirmed that no significant channel incision or aggradation has occurred at this
site (Carson, 2006).
Sediment described and retrieved from the
coring was used to identify relict channel geometry and bankfull stage. Point-bar sediment is
typically a dark red-gray and is characterized
by fining-upward tendencies, with grain sizes
ranging from cobbles to sand (only particles
smaller than pebbles were retrieved by soil
probe, whereas the Vibracore could retrieve particles as large as small cobbles). Overbank sediment is distinctly finer grained, and is typically
composed of laminated silt and clay. Channel-fill sediment is also typically finer grained
than point-bar sediment, but may range from
laminated silt and clay to horizontally stratified
sand. Abundant organic material, ranging from
fibrous peat to twigs, leaf litter, spruce cones,
and isolated logs, is found in channel fills and in
basal point-bar positions along channel margins.
Geological Society of America Bulletin, September/October 2007
Uinta Holocene bankfull flood
Figure 2. Aerial photograph
of the southern portion of
West Fork Black’s Fork
meadow (WFB from Fig. 1),
showing 7 of the 15 paleochannels cored in this meadow.
Channels are identified by
dashed lines marking the
cutbank of the paleochannel.
Solid line in southwest corner
marks the coring transect
shown in Figure 3.
0
50
Estimation of Paleobankfull Discharge
100 m
SE
NW
Distance (m)
0
5
10
15
20
25
0
ground
surface
silt and clay
Depth (m)
1
m. gray sand
pebbles and
cobbles
The clear visual difference between the red-gray
sandy point-bar sediment and the darker finergrained channel-fill and overbank sediments
facilitated accurate identification of paleochannel geometries. Channel-bottom sediment is
composed of cobbles and boulders where relict
channels are exposed in modern cutbanks. This
association is similar to modern channel beds in
the area. Bankfull stage in the relict abandoned
channels was identified as the sedimentological
break between the top elevation for sandy lateral-accretion sediment and the overlying silty
vertical-accretion sediment. The cross-section
area of each channel was calculated on the basis
of reconstructed channel geometry and identified bankfull stage (Fig. 3). An average of 18
soil probes per channel was used to determine
the geometry of each paleochannel and adjacent
point bar and cutbank. Bankfull width/depth
ratios were similar for abandoned channels that
were cored and modern meandering channels
that were surveyed.
silt and peat
pebbles and cobbles
2
boulders
boulders
bankfull cross-section area = 6.48 m
A relation between the 2.0 yr discharge
and bankfull cross-section area for the northern Uinta Mountains was established using
data from U.S. Geological Survey gage stations (Table 1). Discharge measurements taken
roughly every month at active stations were
acquired for 17 gage stations. These data show
actual values for discharge and cross-section
area for a wide range of discharge values, which
were distilled to at-a-station equations relating
discharge and cross-section area particular to
each station (Table 2). For each site, the discharge for the 2.0 yr flood was estimated by the
log Pearson type III method (Interagency Advisory Committee on Water Data, 1986), which
was then applied to the at-a-station equations
to define the cross-section area associated with
that discharge at the gaging site. This establishes the downstream relationship between the
2.0 yr discharge (Q) and the cross-section area
(CSA), as shown in Figure 4. This relationship
is described by:
Qbkf = 2.142 (CSA)0.854,
2
3
Figure 3. Cross-section view of a typical paleochannel used in this study. Horizontal and
vertical distances are measured from an arbitrary datum used to survey ground surface.
Soil probes, shown by vertical gray bars, were generally spaced 1 m apart, except in cases
where the paleochannel profile was changing rapidly, such as at the paleocutbank at 7.5 m
horizontal distance. Lateral-accretion point-bar sediment, channel-fill sediment, and vertical-accretion overbank sediment were identified in the soil probes. Bankfull stage was
selected based on the maximum elevation of lateral-accretion sediments (horizontal line).
The close spacing of cores relative to the width of the channel allows for accurate estimation
of bankfull cross-section capacity.
(1)
where Qbkf is the 2.0 yr discharge (in cubic meters
per second), and CSA is the bankfull cross-section area (in square meters). With this equation,
87% of the variance in the bankfull discharge is
explained by the cross-section area of the associated bankfull channel. Applying this relation to
the gage data shows that the average of the deviations between the 17 observed discharges and
the 17 predicted discharges is 22% and the corresponding standard deviation is 15%. Complete
Geological Society of America Bulletin, September/October 2007
1069
Carson et al.
at-a-station and downstream hydrologic relations for the Uinta Mountains were provided
in Carson (2003). While recognizing that some
inherent variability through time is derived from
physical conditions such as riparian vegetation,
channel sinuosity, and sediment load, beyond
the direct role of bankfull discharge in determining cross-section area, equation 1 can be used to
provide a suitable estimate of the bankfull discharge for any modern or ancient channel based
on its bankfull cross-section area. This provides
bankfull discharge for each of the cored abandoned channels. One assumption of this process
is that the stream channels in the study area have
maintained a meandering planform throughout
the Holocene, rather than converting to a braided
pattern as a result of temporal variations in discharge and sediment load. Studies in the Uinta
Basin to the south of the Uinta Mountains have
found that the Duchesne River has significantly
narrowed and altered its planform over the past
century in response to municipal diversions (see
summary in Schmidt et al., 2005). However,
the width/depth ratios of the cored paleochannels do not vary significantly (p < 0.01) from
those observed for the modern channels in the
study area (Carson, 2003). The apparent lack
of channel adjustment in the studied reaches as
compared to the Duchesne River may reflect the
extent of drainage diversion in the Uinta Basin.
Schmidt et al. (2005) documented decreases
in the 2 yr discharge on Duchesne River by as
much as 60% over the past century, which are
likely far greater than climate-derived streamflow variations. The meandering planform that
has been maintained through the Holocene in
these subalpine meadows probably results in
similar hydrologic relations for both ancient and
modern channels.
Age of Channel Abandonment
The ages of the abandoned relict channels
were determined by two methods. Where possible, ages were determined directly by radiocarbon dating of wood or peat at the base of the
filled channel (Table 3). For the remaining channels, the ages were determined indirectly by
regressing the ages of the directly dated channels against the thickness of vertical-accretion
sediment on the point bars of the relict channels
(Fig. 5). Oakfield and Vibracore cores across the
channel and floodplain study sites showed that
channel bed elevations have remained nearly
constant throughout the Holocene. Therefore,
the accumulation of vertical-accretion sediment
on relict point bars over time scales of thousands
of years has a linear tendency expressed as:
Age = 95.05 (VAS) – 1443,
1070
(2)
TABLE 1. U.S. GEOLOGICAL SURVEY GAGE DATA FOR THE NORTHWEST UINTA MOUNTAINS
Stream
USGS*
Drainage area Gage datum
Annual duration series
2
gage no.
(km )
(m asl)
Period of record
years
(A.D.)
Henry's Fork
09226000
145
2543
1943–1972
30
Henry's Fork
09228000
627
2171
1942–1954
13
Burnt Fork
09228500
137
2530
1944–1983
40
Burnt Fork
09229000
189
2164
1930–1942
13
Smith's Fork
09221500
497
2082
1942–1957
16
East Fork Smith's Fork
09220000
137
2582
1940–1999
60
West Fork Smith's Fork
09220500
96
2627
1940–1981
42
Black's Fork
09217900
337
2686
1938–1939, 1967–2000 36
Black's Fork
09218500
394
2595
1940–1998
59
Black's Fork
09222000
2126
1945
1938–1983
46
Black's Fork
09224700
8029
1868
1962–2000
39
West Fork Duchesne River 09276600
215
2098
1990–2000
11
Rock Creek
09279000
381
2210
1938–2000
63
Bear River
10011500
445
2428
1943–2000
58
Bear River
10020100
1948
1970
1962–2000
39
North Fork Provo River
10153800
63
2280
1964–1996
33
Weber River
10128500
420
2024
1905–2000
95
Note: asl—above sea level.
*Data source: U.S. Geological Survey, http://water.usgs.gov.
TABLE 2. AT-A-STATION EQUATIONS FOR ESTIMATING BANKFULL CROSS SECTION
AREAS (CSA) CORRESPONDING TO THE 1.58 yr DISCHARGE (Q)
2
Stream name
USGS gage no.* At-a-station equation
R
Sample size
1.729
Henry's Fork
09226000
CSA = 0.308 (Q)
0.96
56
1.549
Henry's Fork
09228000
CSA = 0.271 (Q)
0.69
52
1.603
Burnt Fork
09228500
CSA = 0.346 (Q)
0.90
89
1.339
Burnt Fork
09229000
CSA = 0.205 (Q)
0.83
60
1.451
Smith's Fork
09221500
CSA = 0.326 (Q)
0.83
75
1.489
West Fork Smith's Fork
09220500
CSA = 0.304 (Q)
0.84
12
2.904
East Fork Smith's Fork
09220000
CSA = 0.047 (Q)
0.89
13
1.501
Black's Fork
09217900
CSA = 0.345 (Q)
0.91
128
1.398
Black's Fork
09218500
CSA = 0.298 (Q)
0.93
106
1.347
Black's Fork
09222000
CSA = 0.250 (Q)
0.96
67
1.267
Black's Fork
09224700
CSA = 0.297 (Q)
0.90
218
1.549
Duchesne River
09276600
CSA = 0.276 (Q)
0.77
97
1.258
Rock Creek
09279000
CSA = 0.209 (Q)
0.77
179
1.588
Bear River
10016900
CSA = 0.258 (Q)
0.96
136
1.346
Bear River
10020100
CSA = 0.151 (Q)
0.85
127
2.019
North Fork Provo River
10153800
CSA = 0.256 (Q)
0.87
151
1.911
Weber River
10128500
CSA = 0.130 (Q)
0.91
222
*Data source: U.S. Geological Survey, http://water.usgs.gov.
where Age is the basal age of the dated channels
(in calendar yr B.P.) and VAS is the thickness of
the vertical-accretion sediment on the point bars
(in centimeters) (Fig. 5). One would expect that
the accretion rate is likely greatest immediately
following channel abandonment when the floodplain surface is at its lowest, and slows as the age
of the point bar exceeds several thousand years.
However, Knox and Daniels (2001) summarized radiocarbon dates from 53 relict channels
and demonstrated a linear relationship between
age of channel abandonment and thickness of
overbank alluvium over the past ~10 k.y. in the
Upper Mississippi Valley. The assumption of a
nearly constant overbank sedimentation rate was
extrapolated to this study, and a linear relation
was used to approximate channel abandonment
age based on the thickness of vertical-accretion sediment for the period of data in the Uintas. With this equation, ~96% of the variation
in channel age is explained by the thickness of
vertical-accretion sediments accumulated on
the point bars. The standard error of estimate
is ~690 yr. The relatively small standard error
of estimate suggests that equation 2 provides a
reasonable age assignment for relict channels
for interpreting flood behavior at multimillennial
time scales. Application of this equation to the
Geological Society of America Bulletin, September/October 2007
Uinta Holocene bankfull flood
2.0-year discharge (m3/s)
100
y = 2.142 x
R 2 = 0.87
Syx = 1.28
p < 0.001
collected from the WFB meadow identified the
spatial distribution and thickness of this historic
sediment (Carson, 2003), allowing that horizon
to be removed from consideration when calculating thickness of Holocene overbank sediments at the affected paleochannel sites.
0.854
Estimation of Modern Bankfull Discharge
at Each Coring Site
10
1
10
1
100
Channel cross-section area (m2) for 2.0 yr discharge
Figure 4. Downstream relation between the 2.0 yr discharge and bankfull cross-section
area. Data compiled from U.S. Geological Survey gage stations in the northwestern Uinta
Mountains (see Tables 1 and 2). In the figure equations, the terms are as follows: y is the
dependent variable for the regression equation, R2 is the correlation coefficient, Syx is the
standard error of the estimate, and p is the p-value.
TABLE 3. BASAL RADIOCARBON DATES FOR RELICT ABANDONED CHANNELS
†
14
Sample number
Channel
C age
Calendar years B.P.
Material
‡
§
identification
(yr B.P.)
(2σ)
BETA-163182
BBM-1
2720 ± 70
2960–2740
Wood
BETA-163184
BSM-1
760 ± 40
730–630
Peat
BETA-163185
BUP-1
1320 ± 70
1330–1070
Peat
BETA-163187
CHM-4
130 ± 60
300–0
Peat
BETA-163188
EFB-3
350 ± 60
520–290
Peat
BETA-153929
WFBF-1
7080 ± 40
7960–7820
Wood
BETA-154583
WFBF-2
7220 ± 70
8170–7930
Peat
BETA-153933
WFBF-3
630 ± 40
660–540
Wood
BETA-154586
WFBF-4
6470 ± 80
7500–7250
Peat
BETA-163190
WFBF-8
1610 ± 80
1700–1320
Wood
BETA-163191
WFBF-10
830 ± 60
910–600
Peat
BETA-164359
WFBF-11
8970 ± 70
10,230–9900
Wood
†
Beta Analytic, Inc.
Three-letter prefix for channels same as meadows identified in Fig. 1.
§
Calibrated to calendar years from Stuiver et al. (1998, 2002).
‡
undated channels with known thicknesses of vertical-accretion sediment on the buried point bars
allows estimation of ages for those channels.
Accumulations of vertical-accretion sediment
accelerated by human activity after ca. A.D.
1870 were not included in the formulation of
equation 2. During the period ca. 1870–1915,
the north slope of the Uinta Mountains was
clearcut episodically for railroad ties (Baker
and Hauge, 1913). Clearcut logging practices
have been shown to favor increased rates of
slope erosion (e.g., Swanston and Swanson,
1976; van Heeswijk et al., 1996; Shaw and
Julin, 1997). The valley morphometry of the
West Fork Black’s Fork meadow (site WFB in
Fig. 1) makes this area particularly susceptible
to increased rates of overbank alluviation in
response to increased sediment supplies. The
floodplain in the upper portion of this meadow
is covered by as much as 40 cm of visually distinct reddish-brown silt and clay. Analyses of
vertical-accretion sediment in this meadow for
concentrations of 137Cs (e.g., Ely et al., 1992)
identified a peak at ~16–20 cm depth, which
corresponds to ca. A.D. 1950. This suggests
that the entire package of sediment is related
to the historic logging activity (Carson, 2006).
More than 200 Vibracore and Oakfield samples
The relict channels for which paleodischarge
values have been reconstructed represent a
range of drainage areas. To facilitate comparisons of these data, the paleodischarge values
have been standardized by relating them to the
modern bankfull discharge for the same drainage area at the same sites. To avoid any localized
influence on channel size (e.g., bank failures,
fallen trees, slumping of valley-wall sediment
into the channel), a general relation of modern
bankfull cross-section area to drainage area was
established that could be applied to any location
in the northern Uintas. We surveyed bankfull
cross-section areas for 21 modern channel sites
representing drainage areas between 12 and
280 km2 (Fig. 6). During channel surveys, individual measurements to construct the modern
channel geometries were taken 0.5–1.0 m apart
across the width of the channels, providing the
same level of data for identifying modern channel geometry as collected with Oakfield cores
for identifying paleochannel geometries. Bankfull stage was determined as the sedimentological break between sand and fine-grained sediment on the point bar. Where necessary, shallow
trenches were dug on the point bars to accurately
identify the elevation of the sedimentological
break between lateral-accretion point-bar sediments and vertical-accretion overbank sediments. While this method does not necessarily
conform to typical field methods of identifying
channel bankfull geometries, it does standardize the methods used for identifying modern and
paleochannel geometries in this study. The relationship between the cross-section area and the
drainage area is expressed by:
CSA = 0.868 (DA) 0.477,
(3)
where CSA is the bankfull cross-section area (in
square meters), and DA is the associated drainage area (in square kilometers). Figure 6 shows
that the size of the drainage area accounts for
93% of the variance in the bankfull cross-section area. Equation 3 was used to estimate the
respective bankfull cross-section area for the
modern channel with a drainage area equivalent
to the drainage area for a corresponding abandoned relict channel that was cored. This indirect
method provides a better estimation of bankfull
Geological Society of America Bulletin, September/October 2007
1071
Carson et al.
9000
8000
Channel age (calendar yr B.P.)
cross-section area than directly surveying the
modern channel adjacent to each abandoned relict channel (Knox, 1985; Wharton et al., 1989;
Wharton, 1992). The magnitude of the modern
bankfull discharge at each site is then calculated
by application of equation 1.
This methodology provided paired values of
bankfull discharge for each of the 30 abandoned
relict channels and the modern channel with
an equivalent drainage area at the same site.
To remove the dependence of 2.0 yr discharge
on drainage area and to provide for dimensionless comparisons of changes between sites, the
magnitude of a bankfull discharge from each of
the 30 coring sites is expressed as a percentage
deviation from the magnitude of the modern
bankfull discharge at that site.
Calculated bankfull paleodischarges range
between 15% larger than modern and 20% less
than modern (Fig. 7). The propagated error
associated with the application of the linear
regression models averages ~±6%. Much of this
error is associated with the application of equation 1 to the paired values of ancient and modern
bankfull cross-section area for each site. A basic
premise used in this study is that variations in
bankfull discharge are the principal control on
bankfull cross-section area. Therefore, the data
were compiled as paired values of ancient and
modern bankfull cross-section areas, and then
converted to bankfull discharges via application
of equation 1. This methodological step allowed
the final data to be presented as percentage
deviation from the magnitude of the modern
bankfull flood, rather than as percentage deviation from the magnitude of the modern bankfull
cross-section area.
7000
6000
5000
4000
2000
1000
0
0
50
100
150
Floodplain sediment thickness (cm)
Figure 5. Relation between thickness of floodplain sediments and age of abandoned relict
channels in the northern Uinta Mountains. Floodplain sedimentation is the measured thickness of vertical-accretion sediments overlying the lateral-accretion point-bar sediments.
The regression represents a long-term average rate of floodplain vertical accretion that is
~0.013 cm/yr. Radiocarbon dates were associated with wood buried in lateral-accretion sediments underlying vertical-accretion sediments and in basal paleochannel fill sediments. In
the figure equations, the terms are as follows: y is the dependent variable for the regression
equation, R2 is the correlation coefficient, Syx is the standard error of the estimate, and p is
the p-value.
RESULTS AND DISCUSSION
Cross-section area (m2)
for 2.0 yr discharge
50
The modal flood is produced by spring snowmelt in ~95% of the gage years at gage stations within the research area (Carson, 2003).
The very few years that summer precipitation
produces the peak annual flood coincide with
years of below average snowpack and snowmelt
floods; these are cases of smaller than typical
nival floods rather than extremely large latesummer floods. This pattern indicates that for
the north-slope Uinta Mountains, the maximum
flood discharges that are produced by summer
weather patterns are significantly smaller than
the average flood discharges produced by runoff from the winter snowpack. It is, therefore,
the depth of the winter’s snowpack and the rate
at which it melts in the spring and early summer that determines the magnitude of the modal
flood in the Uinta Mountains. This relationship
probably has persisted throughout the Holocene;
thus, temporal changes in the amount of snow
1072
y = 95.05 x - 1443
2
R = 0.96
Syx = 686
p < 0.001
3000
y = 0.868 x
2
R = 0.93
Syx = 1.27
p < 0.01
0.477
10
1
10
100
500
2
Drainage Area (km )
Figure 6. Relation between bankfull cross-section area and drainage area for northwestern
Uinta Mountains drainage basins. In the figure equations, the terms are as follows: y is the
dependent variable for the regression equation, R2 is the correlation coefficient, Syx is the
standard error of the estimate, and p is the p-value.
Geological Society of America Bulletin, September/October 2007
Uinta Holocene bankfull flood
(?)
(?)
(?)
modern channel adjacent to these relict channels
was surveyed in 2001. The measured modern
bankfull cross-section area is 31% smaller than
the expected cross-section area by application of
equation 2, which supports our hypothesis. Two
additional channels show bankfull-flood discharges between 35% and 44% greater than the
modern bankfull flood. One channel is directly
dated as 1320–1700 cal yr B.P., and the other is
indirectly estimated to 2360 cal yr B.P. by application of equation 2. While both ages are within
a period of persistently large floods (Fig. 7),
the extreme values calculated are questionable.
Both of these channel cross sections are located
within tight meander bends visible on floodplain
surfaces, and it is likely that the cross-section
geometries have been influenced locally by
anomalous cutbank erosion (Knox, 1985).
Meyer et al. (1995)
Munroe and Laidlaw (2002)
Magnitude of bankfull flood as a percentage
change from modern bankfull flood
20
15
10
5
0
-5
-10
-15
-20
Average 95% confidence interval
for flood magnitude
Average 95% confidence interval
for indirectly dated channels
-25
12,000
10,000
8000
6000
4000
2000
0
Comparison of Uintas Paleoflood Record to
Regional Paleoclimate Data
Calendar Years B.P.
Channel dating method:
= direct (radiocarbon);
Relative forest fire frequency:
= indirect (use of equation 3)
= frequent;
= infrequent
Figure 7. Long-term variations in the magnitude of the bankfull discharge in the northern
Uinta Mountains expressed as a percentage variation from the magnitude of the modern
2.0 yr discharge. Closed squares represent channels where the age was determined directly
from radiocarbon dating of basal channel-fill sediments; open squares represent channels
whose age was determined indirectly through application of equation 3 (see text). Relative
frequency of forest fires was determined by analysis that quantified density of charcoal
particles and sediment grain size in a sediment core from a lake in the Bear River drainage
(Munroe and Laidlaw, 2002) and dating alluvial fan sediments in the Yellowstone, Wyoming, region (Meyer et al., 1995). The late Holocene peak in bankfull flood magnitudes at
2800–1000 cal yr B.P. roughly correlates to a period of almost no forest fires; these are both
interpreted to reflect cool moist climatic conditions. In contrast, the preceding and following
periods of smaller than modern bankfull floods occurred during more frequent forest fires.
This is interpreted to reflect warmer, drier climatic conditions during these times.
delivered to the Uinta Mountains and the rate at
which it melts in the spring and early summer
are the principal cause of temporal variations in
the magnitude of the modal flood.
The reconstructed magnitudes of Holocene
bankfull floods for the northern Uinta Mountains (Fig. 7) show that bankfull flood magnitudes have varied systematically (nonrandomly)
through time. Former bankfull-discharge magnitudes in north-slope watersheds of the Uinta
Mountains were apparently as much as 15%–
20% smaller than modern ca. 500–1000 cal yr
B.P. and again ca. 2800–5000 cal yr B.P. In
contrast, bankfull-discharge magnitudes were
as much as 10% larger than modern at 1000–
2800 cal yr B.P. and were as much as 10%–15%
larger than modern at 5000–8500 cal yr B.P. A
few problematic data points have been excluded
from the reconstruction of bankfull discharge.
Three channels, located near each other along
the East Fork Black’s Fork, all indicated
bankfull discharges more than 40% smaller
than modern. Two of these channels dated to
ca. 400–450 cal yr B.P., and the third dated
to 3100 cal yr B.P. While these coincide with
the periods of smaller than modern floods, the
extremity of the values is questionable. These
three channels are all located immediately north
(downstream) of where the Mississippian Madison Limestone intersects the valley floor; we
hypothesize that recharge into the aquifer has
depleted the surface-water flow in the stream
and pervasively reduced the bankfull discharges
at these locations. A single cross section of the
The temporal pattern of variations in bankfull-flood magnitudes (Fig. 7) corresponds with
changes in climate during the middle and late
Holocene determined independently by a range
of paleoclimate proxies. The two most recently
abandoned channels in this study indicate that
the floods were smaller than modern during the
past ~400 yr. Dendrohydrologic reconstructions
of streamflow in the southern Uintas (Carson
and Munroe, 2005) and for the entire Upper
Colorado River Basin (Woodhouse et al., 2006)
indicate that persistent drought conditions prevailed from A.D. 1750 to 1900, and possibly
earlier. Meyer et al. (1995) inferred from limited evidence for lateral channel migration and
floodplain widening that the Little Ice Age in the
Yellowstone region was relatively dry; a paucity
of fire-related sedimentation in the Yellowstone
region also suggested this time period was relatively cold there. The correspondence between
these data from the Uintas and Yellowstone with
the current paleoflood record over the past several hundred years suggests that the Holocene
paleoflood data will accurately describe flood
regimes from earlier time periods.
Further comparisons between the Holocene
paleoflood record and paleoclimate proxies
from the Uintas reinforce this idea. Growth
curves of Rhizocarpon geographicum lichen
from the headwaters of Black’s Fork tributaries (Fig. 1) indicate no Little Ice Age glacial
advances in the area (Munroe, 2002). The stability of lichen-supporting rock surfaces during
this period likely indicates insufficient moisture
supply to the Uinta Mountains for a renewed
glacial advance, a conclusion also supported by
evidence of drought inferred from a tree-ring
record on the south flank of the range (Carson
Geological Society of America Bulletin, September/October 2007
1073
Carson et al.
and Munroe, 2005). Numerous studies in the
region corroborate the occurrence of small
floods through 1000 cal yr B.P., coincident with
the occurrence of the Medieval Climatic Anomaly. Submerged tree trunks in growth position
from Jenny Lake in Grand Teton National
Park date to 600 cal yr B.P. and suggest that
water levels at that site were low at that time in
response to warm, dry conditions (Stine, 1998).
Similar stumps in Mono Lake (Stine, 1990,
1994), Owens Lake (Stine, 1998), and Walker
Lake (Stine, 1998) on the east flank of the Sierra
Nevada reflect contemporaneous lake lowstands. In western Nebrasaka and eastern Colorado, the most recent of several Holocene periods of eolian dune mobilization began between
ca. 1000 and 700 cal yr B.P. (Madole, 1994).
Gridded drought reconstructions for most of the
western United States confirm regional drought
conditions during the Medieval Climatic Anomaly (Cook et al., 2004).
Lichenometric evidence for late Neoglacial
readvances in the headwaters of Black’s Fork
tributaries agrees with the paleoflood record
of large floods between 2800 and 1000 cal yr
B.P. (Munroe, 2002). A continuous core from
Henry’s Fork (Fig. 1) produced Picea/Pinus
ratios that indicate that the middle Holocene
in the Uinta Mountains was characterized by
warm and dry conditions with mean annual temperature ~1 ºC above modern, with maximum
temperatures between 6500 and 5400 cal yr
B.P. (Munroe, 2003). Climate cooling began
ca. 5400 cal yr B.P., and between ca. 5000 and
2800 cal yr B.P., bankfull floods were generally
smaller than modern floods of the same recurrence frequency (Fig. 7). Fossil climate proxy
data indicate a return to more cool and moist
conditions that favored larger than modern nival
floods by ca. 2800 cal yr B.P. (Fig. 7). In general, during the middle and late Holocene, warm
and dry conditions have favored smaller than
modern floods, whereas cool and wet conditions
have favored larger than modern floods.
Proxy data for forest fire frequency in the
northern Uintas further support the association
of smaller than modern bankfull floods during
periods of warm dry climate and larger than
modern bankfull floods during periods of cool
moist climate. Munroe and Laidlaw (2002)
identified periods of relatively frequent and
infrequent forest fires over the past 5500 yr in
the northern Uintas on the basis of density of
charcoal in a lake sediment core. The study site
is at Lily Lake, at the confluence of the East and
Hayden Forks of Bear River, within the present
study area. Lily Lake is located within dense
lodgepole pine cover, where infrequent standreplacing fires tend to occur during drought
conditions. These data suggest that forest fires
1074
occurred relatively frequently during 1500–850
cal yr B.P. and 5500–2900 cal yr B.P., but infrequently during 2900–1500 cal yr B.P. (Fig. 7).
Although the changes in flood regime and fire
frequency do not correspond exactly, there is
nevertheless a strong tendency for fires to be
infrequent during the period of larger than modern bankfull floods and more frequent during
periods of smaller than modern bankfull floods.
Evidence for infrequent fires between 2900 and
1500 cal yr B.P. agrees closely with a period of
larger than modern floods during 2800–1000
cal yr B.P., and reinforces an interpretation that
this was a period of cooler, moister conditions
than at present. Similarly, periods of frequent
forest fires from 5500 to 2900 cal yr B.P. overlap
periods of smaller than modern floods between
5000 and 2800 cal yr B.P. This overlap supports
an interpretation that the Uinta Mountains had
a warmer, drier climate than at present during
those periods.
The Uinta paleoflood record corresponds
similarly to the Holocene record of wildfire and
alluvial sedimentation in the Yellowstone, Wyoming, region (Meyer et al., 1992, 1995). The
temporal frequency of forest fires was determined using numerous radiocarbon dates associated with valley-margin alluvial fan deposits.
While this approach has produced a higher-resolution record showing more subtle variations in
landscape response to climate variability, there
is general agreement between the Uinta paleoflood record and the Yellowstone wildfire record
(Fig. 7). The data of Meyer et al. (1995) show
three clusters of infrequent fire-related events
between ca. 3000 and 1300 cal yr B.P., roughly
coincident with the most recent period of larger
than modern floods in the Uintas. Fire-related
events were more frequent between ca. 1300
and 700 cal yr B.P., during the recent period of
smaller than modern floods in the Uintas. The
earlier portions of the Holocene show less correlation between the two records, although this
may be partially due to the decrease in data
points in both records during this time.
Evidence for persistently larger than modern floods during the apparently warm and dry
early to middle Holocene presents a paradox
when compared to the middle and late Holocene climate and flood relations described
here. Munroe’s (2003) pollen study contributes unique paleoclimate data specific to the
Uinta Mountains. Fossil pollen ratios indicate
that the Uintas were as much as 1 ºC warmer
than modern through much of the early Holocene, and that warm, dry conditions persisted
throughout the early to middle Holocene, with
maximum warmth occurring at 5400 cal yr B.P.
For regional comparison, Fall et al. (1995) identified a warm mid-Holocene interval in parts of
the Wind River Range, Wyoming. Their data
suggested that temperatures remained as much
as 1.0 ºC above modern from 12,600 to 3200
cal yr B.P. (10,600–3000 14C yr B.P.), and that
maximum warmth and aridity occurred ca. 6200
cal yr B.P. (5400 14C yr B.P.). Whitlock and
Bartlein (1993) differentiated between summerdry and summer-wet regions in the Yellowstone
area of northwestern Wyoming. The summerdry areas, which are most similar to the study
area, had warm and dry conditions from 9000
to 5000 cal yr B.P. Feiler et al. (1997) estimated
that warm, dry conditions prevailed in northwestern Colorado from 9100 to 5400 cal yr B.P.
(8100–4600 14C yr B.P.). The general agreement
among these studies indicates that large portions
of the central Rocky Mountains had warm, dry
conditions during the early to middle Holocene,
during which time the Uinta Mountains apparently had larger than modern modal floods.
Influence of Large-Scale Atmospheric
Circulation Patterns on Flood Magnitudes
While precipitation over the Uintas is derived
from both North Pacific winter frontal storms
and late summer eastern North Pacific tropical
cyclones, it is the former that controls the flood
regime of the Uinta Mountains. The accumulation of winter snowpack derived from winter
frontal systems plays a critical role in determining the magnitude of the nival flood. The lowpressure systems that produce winter storms
travel in the middle-latitude upper troposphere
circulation, and are associated with relatively
strong and persistent hemispheric-scale atmospheric patterns over the North Pacific (Pyke,
1972; Hirschboeck, 1985). Dry winters occur
predominantly as a result of a persistent highpressure ridge over the Pacific Ocean along the
west coast of the United States. Frontal systems
then flow to the north of the ridge and enter the
continent over the Pacific Northwest, depriving the central Rocky Mountains of the snowpack-producing winter storms. Conversely, wet
winters occur when the high-pressure ridge is
displaced to the west farther off the west coast.
Under these conditions, a low-pressure trough
in the upper atmospheric circulation develops
over the western United States, the associated
circulation shifts the preferential storm track
farther to the south, and winter storms in the
Uintas occur more frequently.
Climatic trends in North America throughout
the twentieth century coincided with persistent shifts in the prevailing upper atmospheric
circulation patterns (Diaz and Quayle, 1980;
McQuirk, 1982; Bradley et al., 1987). Zonal flow
was relatively common from the 1930s through
1950s, followed by more persistent meridional
Geological Society of America Bulletin, September/October 2007
Uinta Holocene bankfull flood
flow conditions that favor an anchored trough
over the central Rockies (Barry et al., 1981;
Balling and Lawson, 1982). These shifts in prevailing hemispheric upper atmosphere circulation patterns favored increased winter precipitation in the Uinta Mountains and coincided with
a significant shift in the Uinta Mountains flood
regime toward an increased mean annual flood
magnitude and increased frequency of extremely
large floods (Carson, 2003).
Long-term variations in flood magnitudes
through the Holocene likely represent such
shifts in large-scale atmospheric circulation patterns. In particular, we attribute the unexpected
early Holocene association of large floods with
apparently warm and dry climatic conditions to
be a consequence of the effects of the retreating Laurentide Ice Sheet on large-scale upper
tropospheric circulation patterns combined with
amplified seasonality of solar radiation during
the early Holocene. Between ca. 12,000 and
6000 cal yr B.P., perihelion occurred during
the Northern Hemisphere summer, resulting in
increased solar radiation relative to today during
the summer and decreased solar radiation during the winter. In addition, orbital obliquity was
greater during the early Holocene than today
(24.2° at 9000 cal yr B.P. as compared to 23.5°
today). Kutzbach (1981) estimated that at 9000
cal yr B.P., solar radiation was as much as 7%
greater than modern during July and 7% less than
modern during January. The maximum anomalies in seasonal insolation relative to modern conditions occurred between ca. 12,000 and 10,000
cal yr B.P., but the Laurentide Ice Sheet was still
sufficiently large enough to significantly influence large-scale upper tropospheric circulation
and delay the maximum effect of seasonal insolation anomalies on climate in North America at
that time (Kutzbach and Ruddiman, 1993; Webb
et al., 1993). Enhanced summer solar radiation
throughout the central Rocky Mountains has
been cited as the cause for higher temperature
and lower effective moisture during the summer
months of the early Holocene (Thompson et al.,
1993; Whitlock and Bartlein, 1993). However,
the flood hydroclimatology of the Uinta Mountains is strongly dependent on winter conditions,
and resultant snowpack, rather than summer conditions. The presence of a significant ice mass on
the northern portion of the continent during the
early Holocene displaced storm tracks southward, thus steering moist air masses toward the
central Rockies (Webb et al., 1993). Despite the
warmth and aridity prevalent through the summer months of the early Holocene, suppressed
solar radiation during the winter months and an
abundant moisture supply would have allowed
snowpack accumulation to meet or exceed current levels. We therefore hypothesize that a
southward displacement of storm tracks coupled
with cooler winters followed by relatively rapid
and more intense summer warming accounts for
the relatively large floods that characterized the
north-slope Uinta Mountain streams during the
early Holocene.
Our hypothesis regarding the flood-climate
relation in the early Holocene is supported by
observed relations between modern nival-flood
magnitudes and melt season conditions. Comparisons of historical streamflow and climate data
for the Uintas indicate that nival floods are largest
in years in which abnormally cool early spring
temperatures are followed by a rapid temperature
rise in the late spring (Carson, 2005). Cooler than
average early spring temperatures allow thick
winter snowpack to persist later into the melt season than usual, and an abrupt temperature rise in
late spring produces unusually large nival floods.
Enhanced seasonality of solar radiation during
the early Holocene would have increased the
temperature gradient between winter and summer, and fostered conditions similar to those that
currently produce large nival floods.
Southwest versus Uinta Paleoflood Records
Numerous analyses of late Holocene paleoflood chronologies have been conducted in the
southwestern United States (Kochel et al., 1982;
Ely and Baker, 1985; Webb et al., 1988; Enzel,
1992; Ely et al., 1993; O’Connor et al., 1994).
Most of the chronologies in this region have
been constructed from flood slackwater deposits.
Ely’s (1997) synthesis of Holocene paleofloods
in Arizona and southern Utah indicates that large
overbank floods clustered into distinct time periods (Fig. 8B). Ely found that large overbank
floods were relatively frequent between 5800
and 4200 cal yr B.P., and again after 2400 cal yr
B.P. Large floods were relatively rare between
4200 and 2400 cal yr B.P., and were underrepresented between 800 and 600 cal yr B.P., during
the Medieval Climatic Anomaly. Ely attributed
the nonrandom clustering of floods to changes in
regional and global climate that affected hemispheric circulation patterns and the delivery of
regional precipitation. Periods of frequent large
floods were associated with cool, wet climate in
the southwestern United States, whereas periods
of less-frequent large floods were associated
with a warmer and drier climate.
Comparison of the Uintas paleoflood chronology with the synthesis of paleofloods in the
desert southwest (Figs. 8A, 8B) indicates that
the two regions underwent broadly synchronous changes in flood regimes. Frequent large
floods in the southwest between 5800 and 4200
cal yr B.P. roughly correspond to larger than
modern bankfull discharges in the Uintas prior
to ca. 5000 cal yr B.P. Similarly, frequent large
floods in the southwest between 2400 and 800
cal yr B.P. coincide with bankfull discharges in
the Uintas that also were larger than modern.
From 4200 to 2400 cal yr B.P., infrequent large
floods in the southwest correspond with a period
of smaller than modern bankfull discharges in
the Uintas. During the later part of the Medieval
Climatic Anomaly, between 800 and 600 cal yr
B.P., the frequency of large floods in the southwest declined slightly, and bankfull discharges
in the Uintas were smaller than modern.
The broad synchroneity between the nature
of flood magnitudes in the southwestern U.S.
and Uinta Mountains underscores the role of
climate variation as a principal forcing factor
accounting for nonstationarity in the two flood
series. Modern gage and climate data indicate
that the flood hydrologies of both regions are
sensitive to the occurrences of winter frontal
storms. Therefore, we infer that cool, wet periods involving winter storms are very important
contributors to anomalous high frequencies
of large floods in these two regions. Ely et al.
(1993) and Ely (1997) demonstrated that large
floods preserved in sedimentary records in the
southwestern U.S. were most likely caused by
intense winter storms or storms related to dissipating tropical cyclones. Delivery of winter
storms to the central Rocky Mountains and
southwestern U.S., as well as delivery of tropical cyclones to the southwest, is associated with
a strongly developed meridional flow pattern in
the large-scale upper troposphere circulation.
Development of a strong low-pressure system
or trough in the upper atmosphere along or off
the coast of the western United States deflects
winter storm tracks southward. The similarity of
the paleoflood records from the Uintas and the
southwestern U.S. reflects this common source
for flood-generating weather patterns.
Upper Mississippi Valley versus Uinta
Paleoflood Records
Magnitudes of Upper Mississippi Valley
bankfull paleofloods were estimated using the
same procedure employed in the current study,
whereas large overbank paleofloods in the Upper
Mississippi Valley were estimated from the
dimensions of transported cobbles and boulders
found in overbank alluvium (Knox, 1985, 1993).
Holocene floods for small tributaries from a few
to a few hundred square kilometers in southwestern Wisconsin indicate that both bankfull
floods and large overbank floods underwent episodic nonrandom variation over the Holocene. A
period of small floods occurred from ca. 5500 to
3300 cal yr B.P. (Figs. 8C, 8D). Proxy climate
data for the midwestern U.S. indicate that this
Geological Society of America Bulletin, September/October 2007
1075
Carson et al.
20
Magnitude of bankfull flood
as a percentage change
from modern bankfull flood
Figure 8. (A) Uinta Mountains bankfull
paleoflood chronology. (B) Record of large
overbank floods in Arizona and southern
Utah based on slackwater deposits (modified from Ely, 1997). (C) Bankfull paleoflood
chronology from the Upper Mississippi Valley (UMV) identified using the same methodology as the Uintas chronology (modified
from Knox, 1985, 2000). (D) Chronology of
large overbank floods in the Upper Mississippi Valley based on competent depth calculations derived from channel-bed cobbles
deposited on the floodplain surface (modified from Knox, 1993, 2000). The two shaded
areas represent periods of smaller than modern bankfull floods recognized in the Uintas.
A Uinta Mountains
10
0
-10
-20
10,000
8000
6000
115
4000
2000
0
4000
2000
0
4000
2000
0
4000
2000
0
B Southwest U.S.
1076
Number of floods
30
20
10
0
6000
Magnitude of bankfull flood
as a percentage change
from modern bankfull flood
40
C UMV bankfull floods
30
20
10
0
-10
-20
-30
-40
10,000
5
Ratio of Flood Depth (d)
to Bankfull Depth (dbkf)
period had mean annual temperatures ~1.5
ºC warmer than the early Holocene and mean
annual precipitation ~15% less than modern
(Bartlein, 1982; Winkler et al., 1986; Baker et
al., 2002). After ca. 3300 cal yr B.P. magnitudes of both bankfull floods and large overbank
floods in the Upper Mississippi Valley tributaries
increased abruptly. This abrupt increase corresponds with fossil pollen evidence that is indicative of a change to cooler and moister climate
(Bartlein, 1982; Winkler et al., 1986; Baker et
al., 2002). The early Holocene record of floods
in the Upper Mississippi Valley is not well
understood, in part because of a lack of data for
large overbank floods (Knox, 2000). However,
existing data show that prior to ca. 7000 cal yr
B.P. bankfull floods were significantly smaller
than modern bankfull floods (Fig. 8C).
Comparison of the paleoflood record for
the Uinta Mountains with those for the Upper
Mississippi Valley (Fig. 8) suggests a relationship between flood regimes in the two regions.
The low-resolution age control does not allow
detailed correlations, but certain broad patterns are evident. Particularly noteworthy is the
early Holocene, when bankfull floods in the
Uinta Mountains and Upper Mississippi Valley
tend to be out of phase with one another. Prior
to ca. 7000 cal yr B.P., bankfull floods in the
Uinta Mountains were 10%–15% larger than
modern, whereas bankfull floods in the Upper
Mississippi Valley were 15%–30% smaller than
modern. Proxy climate data indicates that the
Uinta Mountains were relatively warm (Munroe, 2003), whereas Knox (2000) suggested that
the Upper Mississippi Valley was responding to
a persistent influx of dry air masses at this time
due to the wasting of the Laurentide Ice Sheet.
Beginning ca. 7000 cal yr B.P. and lasting until
5500 cal yr B.P., the two regions shifted to an
in-phase episode of frequent large floods in both
8000
6000
D UMV large-magnitude floods
4
3
2
1
Ave. 95 percentile
C.I. for age
Ave. 95 percentile
C.I. for flood ratio
0
10,000
8000
6000
Calendar Years B.P.
Geological Society of America Bulletin, September/October 2007
Uinta Holocene bankfull flood
locations. This period coincides with independent records of generally warm and dry climatic
conditions in western North America (although
capable of producing large bankfull floods in the
Uintas, as previously discussed), although it is
possible that the shift to large floods in the Upper
Mississippi Valley reflected the final wastage of
the Laurentide Ice Sheet in northern Canada. In
both regions, relative flood magnitudes declined
beginning ca. 5500 cal yr B.P., followed by a
period of sustained smaller than modern floods
between ca. 5000 and 3000 cal yr B.P. This
decrease in flood magnitudes in both regions
coincides with evidence for increased forest
fire frequency in the northern Uintas (Munroe
and Laidlaw, 2002) and warm dry conditions in
the Midwestern U.S. (e.g., Bartlein et al., 1984;
Winkler et al., 1986). The late Holocene records
show a more complicated relationship, reflecting
increased variability in flood behavior. The Uinta
Mountains show a sustained period of larger
than modern floods from ca. 2800 to 1000 cal yr
B.P. During this time, the Upper Mississippi Valley displays shorter time-scale fluctuations, with
floods approaching modern values from 2800 to
2000 cal yr B.P., then decreasing to 10%–15%
smaller than modern from 2000 to ca. 1300 cal yr
B.P., and finally increasing rapidly to as much
as 20% larger than modern at 1000 cal yr B.P.
Lichenometric data from the Uintas indicate that
the large floods at this time correspond to locally
renewed glacial activity. The final decrease in
flood magnitudes in the Uintas after 1000 cal yr
B.P. corresponds to evidence for warming during
the Medieval Climatic Anomaly that is recorded
both locally and regionally.
Based on the modern hydroclimatology
of the western United States, we tentatively
hypothesize that the patterns of flood behavior
identified in the Uintas and Upper Mississippi
Valley through the Holocene may reflect shifts
in the location and pattern of hemispheric circulation and storm tracks. A persistent long-wave
trough located over the eastern United States
anchored by the melting Laurentide Ice Sheet
would have steered winter storms into the Uintas while simultaneously directing dry arctic air
into the Upper Mississippi Valley, producing the
observed pattern of larger than modern floods
in the Uintas and smaller than modern floods
in the Upper Mississippi Valley. A more welldeveloped meridional circulation pattern during
the early to middle Holocene would have provided flood-producing moisture to both regions.
A persistent zonal circulation pattern would
have steered storm tracks to the south of both
regions, producing the smaller than modern
floods observed during the middle Holocene.
Weak meridional circulation may have favored
the varying flood behavior identified in the two
regions during the late Holocene. This hypothesis requires further testing.
CONCLUSIONS
Reconstructions of bankfull channel cross
sections and discharges across the northern
Uinta Mountains indicate that the average magnitude of the modal flood has varied systematically (nonrandomly) throughout the Holocene.
Bankfull-discharge magnitudes in the Uinta
Mountains were apparently as much as 15%–
20% smaller than modern from ca. 5000 to 2800
cal yr B.P. and again from 1000 to 500 cal yr B.P.
In contrast, bankfull-discharge magnitudes were
as much as 10% larger than modern from 2800 to
1000 cal yr B.P. and were as much as 10%–15%
larger than modern from 8500 to 5000 cal yr B.P.
The record of bankfull discharge for the middle
to late Holocene (ca. 5000 B.P. to present) corresponds well with independent paleoclimate data
for the Uinta Mountains. During this period, the
magnitude of the modal flood is smaller than
modern during warm dry intervals and greater
than modern during cool wet intervals. The
early to middle Holocene record (8500–5000
cal yr B.P.) indicates that bankfull-flood magnitudes were greater than modern bankfull floods
despite regionally prevailing warm and dry conditions, especially during the summer months.
We hypothesize that increased summer insolation and decreased winter insolation during the
early Holocene favored deep snowpacks during
the winter and late and rapid spring snow melt,
which fostered larger than modern nival floods
despite warmer mean annual temperatures. We
also hypothesize that the temporal variations in
flood magnitudes identified in the Uintas and
Upper Mississippi Valley through the Holocene
may reflect variations in hemispheric upper
atmosphere circulation patterns.
ACKNOWLEDGMENTS
This research represents a portion of Carson’s doctoral dissertation conducted under the guidance of Knox
and Mickelson. We thank S. Green (U.S. Geological
Survey [USGS], Cheyenne, Wyoming), L. Herbert
(USGS, Salt Lake City, Utah), and J. Schmidt (Utah
State University) for assistance compiling discharge
measurement notes necessary for this research. We also
thank M. Devito, B. Hess, J. Munroe, N. Oprandy, and
L. Wasniewski for assistance in collecting field data for
this research. Input from J.M. Daniels, D. Douglass, D.
Koerner, M. Rhodes, and M. Wartes greatly improved
the final product. Review comments by G. Meyer and
J. Schmidt improved the manuscript, as did reviews of
an earlier draft by L. Ely, L. Mark, and J. Schmidt. Partial funding for this research was provided by National
Science Foundation grant BCS-0081896; Geological Society of America grants 6672-00 and 6858-01;
Sigma Xi grant-in-aid; the Ashley and Wasatch-Cache
National Forests; the Evjue-Bascom Foundation; and
the Morgridge Distinguished Graduate Fellowship.
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