Fresh-water Impoundments

Abstract
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. Water conditions in an impoundment usually are less stable and are often quite different from those found in comparable natural lakes. The limitations of impounded waters for fish productivity can in part be offset by: (1) Controlling the draw‐off (where such regulation is compatible with the major interests for which the impoundment is created) so that fairly constant water level will be maintained during the nesting season of the most desirable species of fish. (2) By judicious restocking (not overstocking, nor again promiscous stocking with all kinds of species) from hatchery or hatchery ponds with vigorous specimens of the desired species. (3) By the construction where feasible of lateral areas of shallow water of constant level and tributary to the main impoundment, in which adult fish can nest, young fish can find refuge and fish food can be produced. (4) By the use of various devices such as floating nests, floating shelters, the planting of exposed portions of the reservoir during low water. (5) By the artificial enrichment of soil and water with specific substances such as phosphates, seed meal and organic wastes, in such limited areas as seem to justify the expense of these procedures. The pollution hazards to the impoundment and the characteristics of the tributary waters should be considered in advance of construction of the dam. Studies of existing reservoirs have shown that much stagnant water may be expected in most impoundments over 30 feet in depth, and that relatively small amounts of wastes having high oxygen demand and of many wastes from various municipal and industrial sources can become very serious hazards to fish life when poured into reservoirs where water conditions are necessarily less stable and often nearer the critical levels for fish life than in the average natural lake. In the interest of fishing in the stream above the impoundment, within the reservoir, and in the stream below the impoundment attention should be given to the position of the outlet. Considering only the water conditions in the reservoir a draw‐off from the bottom of sufficient magnitude to remove much or all of the stagnant water in the deep part of the reservoir would be preferable. Obviously, this volume of water can not be spared from most impoundments. If the flow from the outlet is small and intermittant these advantages to the reservoir water from a bottom draw‐off decrease and the disadvantages to the stream below the dam become increasingly more evident. Stagnant water drawn from the lower levels of the impoundment can be a serious hazard to the fish in the stream below the dam. The low oxygen content of waters drawn from the lower levels of the reservoir constitutes one of the major hazards of such water. Water drawn from the bottom of the reservoir, or from any of the lower levels during the portion of the year in which water of low oxygen content is found in the deep part of the reservoir, should be properly re‐aerated by passing it over or through a system of baffles if proper conditions for fish life are to be maintained in the stream below the dam. Those interested in fish must not expect any impoundment to be a perpetual source of boundless good fishing. The initial period of high productivity may be very short in some reservoirs, and longer in others but the decline will inevitably come unless man makes some adjustments which will create new food supplies and favorable growing conditions, for fish within the impoundment.

This publication has 0 references indexed in Scilit: