GEO 101 Lecture Notes - Lecture 25: Stream Gradient
24 Jun 2018
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Stream Dynamics
Stream gradient. The stream gradient is the downhill slope of the channel. For
example, a gradient of 10 feet per mile means that the elevation of the channel drops a
total of 10 feet over 1 mile of horizontal distance traveled. Gradients are typically the
lowest at a river's mouth, and highest at its headwaters. The higher the gradient, the
faster the stream flows.
Channel shape and texture. The shape and roughness of the channel also affect the
velocity of the flowing water. A narrow channel that is V shaped or semicircular in cross ‐
section results in faster flow; a wide, shallow channel yields a slower flow because there
is more friction between the water and the stream bed. A smooth channel offers less
friction than a rocky or boulder strewn channel, resulting in faster flow. Thus a stream's ‐
velocity is greatest in a narrow, deep, smooth, and semicircular channel.
Stream velocity. The speed at which a stream flows is called the stream velocity. A
fast river moves at a rate of about 5 miles per hour. The water moves most rapidly in the
middle of the channel, where the water is deepest and friction is minimal. The water
moves at a slower rate along the bed of the channel and the banks, where contact with
rock and sediment (and therefore friction) is greatest. The greater the velocity of a
stream, the greater its capacity to erode and transport earth materials over longer
distances.
Stream discharge. A stream's discharge is the amount of water that flows past a
certain point in a given amount of time. Discharge is usually expressed in cubic feet per
second and represents the product of the cross sectional area of the stream and the ‐
velocity:
Discharge (cf/s) = channel width (ft) × average channel depth (ft) × average velocity
(ft/s)
Discharge generally increases downstream because of additional water that is
contributed from tributaries and groundwater that enter the main channel of flow.
Stream discharges vary according to seasonal and precipitation changes. The rates of
flow, discharge, erosion, sedimentation, transportation, and deposition increase
dramatically during flooding and may be a hundred times greater than normal rates.
Stream Dynamics
Stream gradient. The stream gradient is the downhill slope of the channel. For
example, a gradient of 10 feet per mile means that the elevation of the channel drops a
total of 10 feet over 1 mile of horizontal distance traveled. Gradients are typically the
lowest at a river's mouth, and highest at its headwaters. The higher the gradient, the
faster the stream flows.
Channel shape and texture. The shape and roughness of the channel also affect the
velocity of the flowing water. A narrow channel that is V shaped or semicircular in cross ‐
section results in faster flow; a wide, shallow channel yields a slower flow because there
is more friction between the water and the stream bed. A smooth channel offers less
friction than a rocky or boulder strewn channel, resulting in faster flow. Thus a stream's ‐
velocity is greatest in a narrow, deep, smooth, and semicircular channel.
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Document Summary
The rates of flow, discharge, erosion, sedimentation, transportation, and deposition increase dramatically during flooding and may be a hundred times greater than normal rates. The stream gradient is the downhill slope of the channel. For example, a gradient of 10 feet per mile means that the elevation of the channel drops a total of 10 feet over 1 mile of horizontal distance traveled. Gradients are typically the lowest at a river"s mouth, and highest at its headwaters. The higher the gradient, the faster the stream flows. The shape and roughness of the channel also affect the velocity of the flowing water. A narrow channel that is v shaped or semicircular in cross section results in faster flow; a wide, shallow channel yields a slower flow because there is more friction between the water and the stream bed. A smooth channel offers less friction than a rocky or boulder strewn channel, resulting in faster flow.
