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.