Download base level - G

Adam C. Simon
Ph.D., University of Maryland, 2003
Research Associate
Department of Geology
University of Maryland
College Park, MD 20742
p: 301 405 0235
f: 301 314 9661
e-mail: [email protected]
Stream Flow
The flow of fresh water in channels on
the Earth’s surface has been essential
to the development of topography and
most ecosystems.
Some of the consequences of natural stream flow
present engineering and social challenges with which
we grapple year after year, and have through civilization’s history.
Global Water Budget
Although water in streams
and lakes is not significant
in terms of the global water
supply, it is a significant
component of the fresh
water budget.
Flowing surface water is
critical in erosion and
dispersal of sediment.
p.254
Water Moving Sediment on Mars
High resolution images of the Martian surface further demonstrate
that running water was once present on the red planet’s surface.
The
Hydrologic
Cycle
Development
of Stream
Channels
Once moving water
forms a channel, the
channel erodes
upslope: headward
erosion.
The pattern of the
small streams
(tributaries) that feed
the main stream is
based on the material
being eroded.
Fig. 17.3
Base Level
The ability of a stream to
erode is based on velocity
of water. Velocity is
proportional to slope.
At a sufficiently low slope, streams will run without
eroding: this is called the base level. The ultimate base
level is sea level, although local base levels exist, since
flow paths are seldom one consistent slope.
Drainage Basin and Longitudinal Profile
Fig. 17.5
Change in Profile = Change in Base Level
Fig. 17.18
Base Level Changes
lower base level =
steeper profile;
increase stream erosion
(down cut, incise)
raise base level =
shallower profile;
deposit sediment
Processes involving
Processes involving
base level decrease:
base level increase:
-- sea level drop
-- sea level rise
-- subsidence at mouth
-- uplift at mouth of stream
-- uplift at head of stream
-- subsidence at head
-- draining of lakes
-- creation of lakes
Damming
Base level is constantly in
flux, naturally and
artificially.
Dams create artificial
local base levels, with
shallower slopes
(sediment deposited)
upstream and steeper
slopes (erosion)
downstream.
Water Dynamics
Stream sediment
moves by rolling
along the bed or by
bouncing
(saltation).
Stream Transport and Deposition
200 cm/s ~ 4.5 mi/hr
Stream Erosion
Processes by which
streams erode include:
• scouring
• abrading
• lifting/breaking
• dissolution
Darcy’s Law and Discharge
discharge = area x velocity
If discharge increases
and area is unchanged, velocity
must increase.
Internal Features of Stream Channels
Turbulent water flow leads to the
classic meandering path. In this
configuration, water velocity
through the channel is not equal.
Meandering Streams
Immature
Streams
Streams draining areas of high slope tend to have such heavy
sediment loads and variable flows that they do not have the
chance to develop organized meandering channels. Instead
they are braided streams. By definition they are not long-lived.
Braided Streams
braided stream typical of
those draining melting
glaciers in Alaska
Upper Brahmaputra
river, India, draining the
immense Himalayan
mountains
Alluvial
Fans
Alluvial fans occur in areas where steep slopes meet regions of
essentially no slope. This causes an immediate deposition of all
sediment, resulting in the characteristic fan shape.
Deltas
By fundamental
principle, when a
stream reaches
a large body of
water and
velocity drops,
the material
being
transported is
deposited.
The sediment deposited here is called a delta.
The coarsest-grained material drops out immediately,
and finer grained sediment carries out slightly farther,
creating a characteristic deposit.
Modern Delta
Sediment
Distributaries in deltas periodically shift to
follow the most efficient path to the sea -that with the steepest slope. This is not
allowed to happen in today’s heavilyengineered Mississippi delta.
Delta-related
wetlands are
under stress
from sediment
starvation and
lack of natural
flooding.
p.302
Floodplains
Floodplains form in basins carved from unconsolidated sediment
or soft rocks. The floodplain doesn’t meander like the channel: it is
the broad area of uniform topography around the stream.
Stream Terraces
Fig. 17.21
Like lake terraces and coastal terraces, these indicate earlier
floodplain positions for streams which erode their floodplains
due to changes in base level.
Streams Locked in Valleys
Streams like these have virtually no (ordinary) floodplains. They
have carved into rock so deeply, that their meandering and other
characteristic
evolutionary features are restricted.
Why do some streams erode bedrock like this?
Antecedent Streams
uplift/compressive
deformation
Superposed
Streams
Without knowledge of
the regional geologic
history, differentiating
between an antecedent
and superposed
streams can be difficult.
Drainage Pattern
Evolution
Old superposed channels cut
across regional structures.
Younger channels follow
predictable paths through easily
eroded valley rocks.
Drainage Pattern Evolution
Sedimentary rocks with sedimentary structures that show
paleocurrent direction of streams give us further supporting
evidence for plate movement, as in the evolution of drainage in
Africa and South America in the last 300 Myr.
Watersheds
When one drainage basin erodes into
another, it is stream piracy.
Every stream is defined by an area on the ground where incident
precipitation will all flow into that stream. These drainage basins
or watersheds are separated by topographic highs: divides.
The Continental Divide
In North America, the continental divide
separates watersheds that drain to the west
from those that drain to the east (and the arctic).
The
Mississippi
and its
tributaries
dominate the
North
American
drainage
system.
Western
Hemisphere
Drainage
What happens to water
in these regions?
Most of stream water in the
US drains into the Gulf of
Mexico.
Practically all of the stream
water in South America
drains into the Atlantic.
Do you see why?
The
Riparian
Zone
This is the highly vegetated border along most stream channels,
providing the transition between aquatic and land environments.
Importance of the Riparian Zone
Aside from stabilizing a channel (through root systems), the
riparian zone acts as a filter for land-derived pollutants, and
provides food, shade and habitat for wildlife around the stream.
Conservationists call these riparian buffers.
In natural streams,
floods occur
periodically when
seasonal discharges
grow extremely large
and waters exceed
channel banks.
The first thing that
happens after water
goes over bank is the
velocity drops, and
sediment is deposited.
This makes natural
levees along channels.
Floods
Flood Recurrence Intervals: Not Prediction
A graph such as this
can be constructed
for any stream where
data exist. The more
long-term data
available, the closer
to being ‘predictive’
these become.
Flood Data
Floods and Urbanization
surface runoff vs. infiltration
natural land cover vs. urban area
Houston Flooding, 2001
urbanization
(#4 city in US)
+
near sea level
+
tropical storm season
=
Floods: Good or Bad?
Prior to mass urbanization of floodplains, natural floods were
not a problem to society. We now attempt to control floods
on most large stream systems by regulating flow with dams
and similar structures.
Nonetheless, natural flooding is important:
• sediment deposits on
floodplains contribute to the
formation of nutrient-rich
soils
• floodplain wetlands are
important habitats for plants
and animals
Credits
Some of the images in this presentation come from:
Marshak, Earth: Portrait of a Planet (1st ed)
Hamblin and Christiansen, Earth’s Dynamic Systems (8th ed)
Press and Siever, Understanding Earth (3rd ed)
USGS Bulletin 1471
The Chesapeake Bay Program