The river deposits its sediment load in the impoundment when the flow velocity slows and the particles settle out. Over time, sediment can fill in the impoundment. Eventually, the impoundment may become so shallow that the sediment must be removed by dredging or other means. The river downstream of the dam is starved for sediment because the sediment naturally flowing in the river has been trapped behind the dam. The water flowing through the outlet of the dam may be relatively clear, and carry little sediment.
Water flowing over a dam can cause scour holes to form immediately below the dam.
Scour holes may undercut the foundation of the dam threatening the integrity of the structure. High-energy flow of water over the former Rockdale Dam erodes a scour hole. The quality of water in the river, free flowing or impounded, greatly depends on the types of human activities within the watershed. The chemical composition of water and the relationship of its chemistry to its intended uses, such as support of aquatic life, drinking water, and recreation, determine water quality. Dams not only impound the water of a river, but also impound the dissolved and suspended substances carried in the water.