The slope of the river's surface is its gradient, usually expressed as the drop in feet per mile along the river. Generally, the steeper the gradient of the river, the faster the flow. The thalweg is the part of the stream channel with the fastest flow. Rivers can have riffles, which are characterized by relatively steep gradients and shallow depths, and pools, which are characterized by relatively gentle gradients and greater depths. Riffles are found where coarse sediment has been deposited in the channel.
Pools can form behind natural obstructions in the channel, such as logs deposited on the bed.
As one might expect, a river's flow is faster in riffles than pools. Free-flowing rivers usually have pools and riffles, which provide diversity in habitat types in a river. Morphology and sediment restoration Land-use, such as construction and agriculture, may provide more sediment than a river naturally carries, and this additional sediment can upset natural balances in rivers. There are many ways that people may reduce excess sedimentation either at the various sediment sources, or just before sediment reaches a river or impoundment. Any work that prevents or reduces the amount of sediment entering the river upstream of the impoundment reduces the amount of sediment entering the impoundment.