OCR Text |
Show PLAN OF OPERATION OF THE DAM The value of a reservoir for recreation and wildlife depends almost entirely upon the plan of operation of the dam and the resulting effect on water levels in the reservoir. Power plants are usually better adapted to recreational use and wildlife than irrigation or flood- control reservoirs because a more constant water level is required for operation of the generators at maximum efficiency. Irrigation and flood- control reservoirs may be drained completely during the summer when the recreation season is at its height. As a general rule, however, only some of the smaller irrigation reservoirs are apt to be drained dry each year. In most cases the reservoirs are planned with a conservation or dead- storage pool and usually there is some hold- over storage in irrigation reservoirs. In all reservoirs there will be some fluctuation of water level. Generally, the recreational and wildlife value will be in inverse proportion to the amount of fluctuation in water elevations and areas between maximum, average, and minimum pools and the rate at which the water level is raised and lowered. However, the effects of fluctuations are dependent upon a number of highly variable factors which seldom occur in identical combinations at any two reservoir sites. For this reason, the recreational and wildlife values must be estimated separately for each potential reservoir. Effect of fluctuation in water levels on plants.- Degree and season of water- level fluctuation in reservoirs are fundamental to all other considerations because in the water, as on the land, plant life is the key link in the production of all other forms of life. Aquatic plants, like those on land, require the action of sunlight for the chemical process that builds up food substances in their green tissues. However, sunlight in sufficient strength to permit plant growth seldom can penetrate the water of even the clearest lakes to depths of much more than 50 feet and in some lakes the illumination is reduced by dissolved and suspended material to 1 percent of its surface intensity at a depth of only 10 feet. 6 For this reason, aquatic plant life is confined, essentially, to the shallow, more or less marginal areas of a lake or reservoir, which are pre- 6 Clarke, 1939, p. 29- cisely the areas most drastically affected by fluctuations in water level. In the majority of deep lakes, most aquatic animals also are confined to the surface layers because of the general decrease in oxygen as depths of 100 feet are approached. The relative productivity of a lake is proportional to its area rather than to its volume. Obviously, a sudden drop in the water level of a reservoir can completely destroy the food- producing fringe of aquatic vegetation by leaving it high and dry. A sudden rise can be almost equally destructive by burying the vegetation in the cold, sunless depths. On the other hand, if the drawdown or inundation is slow enough to permit the gradual establishment of a zone of new plants below the shifting water level, the biological disturbance is considerably less severe. In a body of clear water located in a region of high light intensity, the productive zone of vegetation can extend downward to a considerable depth, with the result that even abrupt fluctuations of water level amounting to several feet can be sustained without the loss of the major food- producing zone. Relative transparency of the water, proportion of sunny days to overcast, temperature of the water in relation to plant growth- these and other variables must be evaluated before one can predict the exact effect either of rate or amount of water fluctuation on the aquatic life of a reservoir. In general, however, gradual fluctuations of up to 75 feet do not severely curtail the basic productivity of reservoirs in warm regions having a high light intensity. Lake Mead is a good example of this combination of factors. On the other hand, a similar fluctuation in a cold, mountaintop reservoir having a growing season of only 3 or 4 months probably would result in a biological desert. Even where the extent and rate of water fluctuations do not prevent the establishment of aquatic plants in a reservoir, they have a vital effect on the kinds of plants that can survive there. Plants that multiply and spread very rapidly obviously can advance and retreat with moving water levels by a process of individual replacement better than those that grow and propagate slowly. Microscopic plant forms, like green algae, that coat lake bottoms, submerged rocks, and dock pilings with a green slime or " moss," are able to replace them- 105 |