Download topic #16

Streams:
Transport to the
Ocean
Gary D. McMichael/Photo Researecher
The Hydrologic Cycle
• Describes water’s movement on the surface
• P = RO + ET + I
• Driven by solar heat
– ocean water evaporates
– wind carries moist air over land
• topography can force moist air to higher altitudes
• as air rises, it cools, water condenses to form clouds and
then precipitate as rain or snow (P)
– rain that falls on the land can:
• evaporate back into the atmosphere (E)
• be taken up by plants, which return water to the atmosphere
– called transpiration (T)
• flow over the ground, enter streams and rivers, and
ultimately be returned to the sea; called runoff (RO)
• infiltrates and becomes groundwater (I)
The Hydrologic Cycle
STREAMS
This is the “RO” of
P = RO + I + ET
Streams
Stream : body of water flowing in a
channel
The floor of the channel is called the
bed.
The sides of the are called the banks.
Flood: when bodies of water overflow
their banks and water covers the
adjacent land called the floodplain.
A stream system network.
Drainage basin
Area of land surrounded by
topographic divides in which all the
water is directed to a single point
In Hawaii, divides are steep &
basins are small!
Mississippi River Drainage Basin
Drainage Basin of the
Colorado River
Some terminology
• VELOCITY (V) - DISTANCE PER UNIT TIME
(cm/s, mph)
• DISCHAGRE (Q) Total amount of water that
passes a given point in a stream per unit
time (m3/s) = width (m)  depth (m) 
average velocity (m/s)
In the U.S., this is expressed as cubic feet
per second (cfs):
1 m3/s = 35.9 ft3/s
River at Low
Discharge
River at High
Discharge
Downstream
changes
Longitudinal Stream Profile of
the Platt and South Platt Rivers
Flooding
• Water in the stream is greater than the volume of
the channel.
• Interval between floods depends on the climate of
the region and the size of the channel
• In Hawaii, lots of FLASH FLOODS for mountains
are steep, flood plains are small & there is lots of
CONCRETE!
City Built on a Floodplain
Xie Jiahua/China Features/Sygma
Recurrence interval
Average time between the
occurrences of a given event
The recurrence interval of a flood
of a given size at a given place
depends on:
• climate of the region
• width of the floodplain
• size of the channel
Annual Flood Frequency Curve
More stream terms
competence: measure of the largest
particles a stream can transport,
proportional to v2
capacity: maximum quantity of
sediment carried by stream,
proportional to Q and v
Job of Streams
• Carry away runoff to lakes and seas
• Erode land (degradation)
• Transport and deposit sedimentary
debris
JOB - TO ERODE
• STREAMS CUT, DEEPEN & WIDEN VALLEYS
• BY:
–HYDRAULIC ACTION - flowing water
can pick up load
–ABRASION & IMPACT - solid load
wears down stream bed
–SOLUTION - dissolves channel & load
Waterfall Retreating Upriver
Donald Nausbaum
Pebbles
Caught in
Eddies
Form
Potholes
Carr Clifton/Minden Pictures
Stream behavior
• Mostly determined by velocity and
shape of channel.
• These factors combine to allow
either laminar or turbulent flow.
• Turbulent flow is much more
erosive & picks load up.
• Stream velocities may vary from
0.25 to 7 m/s.
Laminar flow
• Smooth sheet-like flow at a low
velocity
• Usually confined to edges and top
of stream
Laminar flow
Turbulent flow
• Irregular swirling flow
• Occurs at most rates of stream flow
• Keeps particles in suspension
Turbulent flow
Laminar to turbulent transition
Laminar flow
ONERA
Turbulent flow
JOB - TO TRANSPORT
• Function of stream V & Q
• Load is the amount of material carried
by a stream in all forms:
–Dissolved load (CATIONS)
–Suspended load (fine grains)
–Bed load (coarse material)
Sediment
Transport
Saltation
Grain Size and Flow Velocity
Lower Velocities Form Ripples
ripple
Higher Velocities Form Dunes
ripples
dune
dune
Giant ripples in the
Channeled Scablands
JOB - TO DEPOSIT
• As V drops, stream begins to
drop its load, coarsest &
heaviest first
• Forms are streams, deltas,
alluvial fans & flood plains
Two important stream types
1. Meandering Streams
• Gentle gradients, fine-grained
alluvium
• Minimizes resistance to flow and
dissipates energy as uniformly as
possible (equilibrium)
• Features: point bars, oxbow lake,
migrating meanders
Meandering River Over Time
Meandering River
Point Bar
Peter Kresan
Meandering stream,
Phnom Penh, Cambodia
Oxbow
Lake
Incised Meanders, Utah
Tom Bean
Two important stream types
2. Braided Streams
• Sediment supply greater than amount
stream can support.
•At any one moment the active channels
may account for only a small proportion
of the area of the channel system, but
essentially all is used over one season.
•Common in glacial, deserts, and
mountain regions.
Braided River
Tom Bean
Brahmaputra
River:
a
braided river
Courtesy NASA
Typical Large Marine Delta
Alluvial Fans
Michael Collier
Parts of a River System
Formation of Natural Levees
BASE LEVEL
Definition is:
LOWEST LEVEL TO WHICH A
STREAM CAN CUT
Think of it as:
Elevation at which a stream
enters a large body of water
such as a lake or ocean
Effects of Building a Dam
Original Profile Graded to Regional Base Level
Effects of Building a Dam
Dam Forms New Local Base Level
Effects of Building a Dam
Deposition Upstream
and Erosion Downstream
Formation of River Terraces
When Base Level Changes
GEOMORPHIC CYCLE
• STAGES OF LANDSCAPE DEVELOPMENT
• HILLS & VALLEYS CUT INTO SIDES OF
VOLCANOES
• STAGES ARE:
–
–
–
–
YOUTH
SUBMATURE
MATURE
OLD AGE
G
E
O
M
O
R
P
H
I
C
C
Y
C
L
E
INITIAL STAGES OF STREAM
EROSION IN HAWAII
• Runoff is difficult to establish for rock is
very porous & slope is steep.
• Surface has to be sealed by chemical
weathering or ash before it happens.
• Happens during Capping Stage
• Streams tend to follow lava flow
margins or former lava rivers.
• It begins at the coast & moves upwards.
Development of an experimental
drainage network - headward erosion
Typical Drainage Networks
Amphitheater-Headed
Valleys
• In Hawai‘i, erosion of the volcanoes
by stream erosion and mass
wasting produces these distinctive
features
• Caused by high rainfall, steep
radial drainage, and alternating
layers strong and weak rocks
dipping seaward
Stream
Piracy
or
Stream
Capture
planezes
Ha‘iku Valley
Giant “SOIL AVALANCH”
Formation of the Nu‘uanu Pali
Graded stream
Stream in which
neither erosion nor deposition
is occurring, due to an
equilibrium of
slope, velocity, and discharge.
THE END