Several features of impoundments and reservoirs, either inherent or the result of operations at the dam, can reduce and limit the biological productivity of an impoundment and consequently the fish production of its waters. Among these characteristics are the stagnation of large masses of water, the removal of food materials in the waters drawn out of the reservoir, the limited lateral areas of shallow water around the margin of the impoundment, extensive fluctuation of water level, and continued and often heavy silting in of the bottom.