FACTORS DETERMINING THE RECREATIONAL BENEFITS OF RESERVOIRS (continued)
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.  For this reason, aquatic plant life is confined, essentially, to the shallow, more or less marginal areas of a lake or reservoir, which are precisely 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 growththese 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 themselves in a few days, and thereby can shift with the waters far more readily than large slow-growing perennial plants of higher evolutionary types like the cattails, pondweeds, and ditch grasses.
Weed growth is deleterious to fish in that the fish-food organisms, to which such growth gives shelter, becomes inaccessible to the fish. As a result, production of fish is less in a pond with weedy vegetation. The effects of fluctuation in a given reservoir obviously are dependent upon many variables. For example, "a short period (two to six weeks) of flooding is usually good for ducks because it promotes growth of lake-margin food plants without injuring those adjusted to normal level. That is if the flooding occurs before the nesting season, or if after the nesting season only lasts a week or ten days. In the case of draw-down, both the rate and the time of year would be important." 
Effect on animals.There will be no large aquatic plants along the margins of reservoirs having water-level fluctuations of more than 10 feet during the growing season. Since these plants comprise the principal food supply and habitat of waterfowl, the absence of these plants seriously curtails the wildlife and recreational values of reservoirs. The role of such relatively barren bodies of water in the conservation of waterfowl will be relatively minor because nesting opportunities will be few. Therefore, such areas will be of little use except as temporary stopping places along the routes of migration. Unfortunately for recreation and wildlife conservation, most of the existing reservoirs in the Colorado River Basin are of the excessively fluctuating type. Stable reservoirs with ample food and cover are all the more critically needed in this region because of the prevailing scarcity of suitable natural areas for waterfowl. Reservoirs that control floods also eliminate many of the ponds and marshes that normally are formed along river bottoms during periods of overflow. Thus, former breeding grounds of waterfowl, muskrats, and other marsh-dwellers are destroyed, and the need of additional stable reservoirs with food and cover becomes all the greater.
Lack of an extensive growth of large aquatic plants does not in itself mean that a reservoir fluctuates so much as to be unsuitable for fish. If the fluctuations are not too violent, and if other growing conditions are favorable, microscopic and filamentous algae may be produced in such enormous quantities that they blanket all the shallow, sunlit depths. This microscopic aquatic forest teems with such "game" as the larvae of midges, mayflies, stone flies, caddisflies, dragonflies, tiny crustacea, aquatic beetles, snails, worms, rotifers and other fish foods.
The other major source of food in lakes and reservoirs is furnished by the billions of microscopic free-floating and swimming plants and animals known as plankton. Though usually invisible to the casual observer, the plant forms sometimes become so abundant under proper conditions of temperature, light, and fertility as to impart a greenish tinge to the water. Smaller animal plankton like the one-celled protozoas, as well as the plant forms of plankton, are eaten by larger plankton forms such as rotifers, or the barely visible crustacea known as "water fleas." These in turn are eaten by the aquatic insect larvae and by small fish (which also consume some of the small plankton directly), and these insect larvae and small fish, plus some plankton, comprise the principal food source for the larger fish. The basic productivity of reservoirs with bare shore lines is dependent partly upon the submerged and largely unseen shallow-water blanket of algae and, even more importantly, upon the invisible plankton swarms.
Terrestrial insects that fall into the water furnish a supplementary source of food for fish during the summer months, but the amount of such food is correlated with the amount of land vegetation near the shore. In the Colorado River Basin most of the reservoirs have bare shore lines of varying extent so that this source of food is relatively unimportant.
Water fluctuations are additionally harmful when they crowd fish into small residual pools where they suffer from oxygen depletion, lack of food and predation.
Spawning beds, and shallows used by fish as refuges from attacks by larger fish, may be drowned out or left dry by such fluctuations. A benefit to wildlife and recreation may result from certain reservoirs if, in addition to reducing destructive floods, the release of water below the dam is so controlled as to augment a normally meager stream flow by providing additional water in late summer and fall.
Reservoir temperatures are a product of the complex interplay of temperatures of incoming streams and of local seasonal climatic conditions. Factors such as the relation of total volume of water to surface area exposed to the air, relative depth of water, interplay of sub-surface currents, volume of incoming water, mixing and oxygenating effects of winds, and the degree and duration of seasonal climatic changes are all variables contributing to the changing subsurface "climate" of reservoirs. Complex stratification of temperature zones in the deeper lakes results from the interplay of these and other factors, and these temperature stratifications are altered more or less radically by the changing of the seasons. Naturally, this emphasizes again the need of making an individual planning study for each reservoir and the difficulty of making general statements as to the effects of reservoir construction.
As previously pointed out, natural lakes and reservoirs in the higher mountains of the Colorado River Basin warm up for only a few weeks during the short summer season. Productivity is low because plant growth ceases at temperatures of around 40°F., the exact temperature varying with the species of plant. Many small organisms go into hibernation for the winter. Trout (and other fish) feed but little at temperatures near 32°F., and become more or less dormant.
Other favorable factors sometimes compensate for the short growing season of the higher altitudes. Abundant light and clear waters may favor the rapid growth of a zone of bottom-dwelling algae or aquatic mosses to a considerable depth, and the development of a large plankton population, as at Crater Lake, Oregon, where fish growth is excellent despite a water surface elevation of 6,177 feet, and summer surface temperatures of between 47° and 63.6°F  Obviously, it is more important that fluctuations in water level be kept at a minimum in reservoirs at the higher altitudes because plant growth, being slower, cannot adapt itself to changing levels as rapidly as at lower altitudes.
At the lower altitudes, the higher temperatures afford a potential growing season that sometimes extends throughout the year, and in desert regions particularly there may be abundant light. However, in the Colorado River Basin, a prevalent adverse factor that more than offsets favorable light and temperature conditions is the presence of large quantities of silt, which cuts out the light, smothers bottom-dwelling food plants and spawning grounds, and suffocates fish. If the reservoir is large enough to provide clear water beyond the area of silt deposition, production of plant and animal life may be extremely vigorous, provided water-level fluctuations are not extreme.
The most favorable growing temperatures for trout under natural conditions appear to be between 50° and 70°F  with 70°F. close to the upper safe limit, and 80°F. too high if sustained for any length of time.  Trout require water of a higher oxygen content, but can endure lower winter temperatures than the warm-water fish, such as bass. The latter require warmer waters for feeding and spawning.  The general life zone requirements of the two types of fish already have been discussed in Chapter I.
Aquatic plants, like those on dry land, require fertile soil containing organic matter, as well as warmth, for maximum productivity. For this reason, many reservoirs in the Colorado River Basin will be unable to compare in lush productivity with reservoirs in other parts of the country that have a richer soil.  The silt, resulting from both natural and man-caused erosion, that collects in so many of the lower reservoirs of the basin, is composed largely of inert subsoil and lacks the required organic matter. Reservoirs in the higher mountains, though more fertile, are handicapped in their productivity by prevailing low temperatures. Thus, in fertility as in other characteristics, it will be seen that few reservoirs in the basin will combine all of the elements necessary for maximum productivity.
Aquatic biologists have almost universally noted that new reservoirs exhibit a relatively high rate of productivity during the first few years following construction, but that this initial productivity eventually drops to a lower and more permanent level. Lake Mead is a good example.
This above-normal production, I believe, can be attributed to the accumulation of organic materials on the floor of the reservoir site which are suddenly brought into the aquatic complex by the new waters as they flood them for the first time. I have noticed this phenomenon in several reservoirs of the Colorado River drainage in Utah and also in waters of California . . . I am (also) of the impression that as soon as the first 'bloom' of production in most high mountain reservoirs is over, the resulting production is not much greater than what might have been expected from the stream before the reservoir was constructed. This assumption is likely to hold in steep-sided, mountainous reservoirs with a minimum of shoal water. Naturally this comparison cannot be made in all instances.