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 (Strahler) 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. Individually they are shorter. Collective (total) 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 surface 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
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