Streams Hydrodynamics
• It is useful to have a system to identify types of
streams by hydrodynamic characteristics.
• Various factors influence the amount of flowing
water (discharge), stream channel shape and flow
dynamics (fluvial geomorphology).
– Watershed size (total land area draining into a stream).
– Watershed topography (geological processes)
– Vegetation cover and land-use (evapotranspiration)
– Soil type (influences on infiltration and percolation)
– Climate (frequency and duration of rains; and
temperature affects on evaporation)
• High precipitation regions, stream flow is more permanent
• Arid regions, stream flow is more ephemeral.
Watershed size vs. discharge volume
Fig. 5.3
Stream Order
– Smallest streams = first order.
– Streams increase in order by 1
only when joined with a stream
of same order.
– Discharge increases with order
number, but also depends on
number of additional tributaries
of lower order.
– Low-order streams are most
common. Collective length of
low-order streams is more than
high-order streams.
– Terrestrial-stream interactions
predominantly occur on small
streams.
Which streams need to be
targets of management?
How does water enter streams?
• Most water enters via groundwater; this is called base flow.
• It takes a lot of rain to create sheet flow (runoff) in most areas.
Base Flow
Stream Discharge
•
Discharge is the volume of water
passing through the stream per unit
time (different from current velocity).
•
Plotting discharge rate vs. time
(hydrograph) can provide a good idea
of stream dynamics:
– Dry areas = high variability within years
– Wet areas = high variation between
years
– Ground water fed = fewer peaks in
discharge; more base flow
– Surface water fed = lots of peaks in
discharge; “flashy”
•
Terrestrial surface influence variability
greater for stream-fed; Groundwater
fed has more stable water quality.
Flooding
• When rain is high enough to create
significant sheet flow, floods often occur.
– Increases discharge rate.
– Can be periodic, cyclic (10 yr flood, etc.).
– Intensity of precipitation, infiltration rate, runoff
rate influence severity of floods in a watershed
– Human activities (urbanization) have increased
flood rates.
The Stream Channel and Stage Frequency
Profile of a Stream
• Three basic types of sections:
–Riffle: Shallow, fast, turbulent water
–Run: Deeper, fast, smooth-surface water
–Pool: Deep, slow area
• Reach: A stretch of stream containing runs,
riffles and pools
• The sediment composition and topography of a
stream can affect the appearance of these
different sections (e.g.: fine sediments, no riffles)
Fig. 5.10
(Dodds, 2002; Fig. 5.11)
In-Stream Flow Profile
Thalweg: Line of highest current velocity.
* In straight streams, thalweg in center.
* In meanders, thalweg lies along the outside of the bend.
(Dodds, 2002; Fig. 5.10)
Stream and River Bends
• High velocity flow at outside of bend
erodes and transports more sediment.
• Low velocity flow on inside of bend
deposits sediments.
• Current velocity profile at outside of bend
creates a downward flow that undercuts
bank.
Erosion at outside of bend and sediment deposition at
inside of bend strengthens and exaggerates the
formation of the bend.
Meanders are a common feature of flowing water. They
are self-organizing and get exaggerated through time.
Larger drainage area; greater
discharge; bigger meanders.
Characteristics of Meanders
• Sinuosity = thalweg : linear distance ratio; S >1.5 is meander.
• The radius of the meander is ~ 11 x channel width.
• Radius is about 1/5 the wavelength.
•Large channel -> large radius -> high discharge.
• On flatter planes where sheet flow is dominant, streams
become braided.
Floodplain
• As a river meanders back and forth
through the valley over geologic time, it
creates a floodplain.
• The floodplain gets inundated with water
following seasonal flood cycles, providing
a riparian wetland habitat.
Floodplain Heterogeneity
• All these different processes (meanders,
velocity profiles, etc.) creates highly
variable system over different spatial
scales in the river / stream valley.
– Natural levees, oxbow lakes, boulders, fallen
logs, vegetation patterns, etc.
– This landscape heterogeneity plays an
important role in flood control and ecology.
Fig. 5.13