FACTORS DETERMINING THE RECREATIONAL BENEFITS OF RESERVOIRS
In the Colorado River Basin, where few lakes occur naturally, the creation of large water bodies is of great recreational importance. This is especially true in the large, arid, lower portions of the basin. The higher portions of the basin are fairly well supplied with clear, year-round streams and attractive small lakes. Here reservoirs play a much less important role in the general recreation picture.
On the whole, a reservoir on a mountain stream will be of less benefit for fishing than one on a lowland stream, at least in the Colorado River Basin, and may be seriously detrimental to fishing. One reason for this is that the mountain streams usually are silt-free, and provide a fair-to-excellent habitat for fish even before construction of the reservoir. Another reason is that in the Colorado River Basin the majority of the potential mountain reservoirs as now planned will have a large drawdown. Thus, fluctuating shore lines, with their drastic curtailment of aquatic food production, are substituted for the relative stability of the original mountain stream condition.
As will be shown, the potential productivity of a reservoir is rather closely limited to the area of shallow bottom that is kept at a favorable depth for plants, rather than by mere volume of water. The productive area of a proposed deep reservoir usually will occur only along the shallow margins, whereas the productive area of the mountain stream that it is to replace usually comprises the entire stream bed from bank to bank. Extreme examples occur, particularly in Colorado, wherein existing fish production and extremely important attendant recreational values on various beautiful mountain streams would be jeopardized by the substitution of relatively barren reservoirs with bleak, fluctuating shore lines. One of the best of such examples is afforded by the Gunnison River, which is one of the finest natural trout fishing streams in the United States. Reservoirs on the upper reaches of this river and its major tributaries would, in most cases, destroy more recreational values than they could supply. On the other hand, reservoirs near the headwaters of these tributaries sometimes bring an increase in production and furnish good fishing, as in the case of the Taylor Park Reservoir near the upper end of the Taylor River. In this case, improvement may have resulted from the substitution of a large body of water with extensive, shallow shore lines for a previously existing small streamway. It illustrates again the need of estimating the effects of each proposed reservoir separately.
Reservoirs in national forest primitive areas and other high mountain country usually add little if anything to the general recreational value of the area and frequently destroy more than they create. They alter natural conditions just as do roads and other facilities. Construction of the dam requires roads for the transportation of men, materials, and machinery. Aggregate or fill for the dam is usually obtained as near as possible to the dam site, eliminating a piece of wilderness. Stockpiles and construction camps take more. Tunnels and canals for transporting the water from the reservoir to the area of use further spreads the effects of the project. The cuts and fills required for highline canals on mountainsides are frequently visible for long distances. This is not all. The mere fact that these works exist eliminates the important intangible value of an area undisturbed by works of man.
There are a number of instances in the Colorado Basin where the economy of the nearby communities is dependent to a large degree on the income from fishermen and vacationists attracted to the region from considerable distances. If the rarer trout stream fishing is replaced by the more common and widespread reservoir fishing, it is likely that the attraction of these regions will be considerably less.
Converting natural lakes into reservoirs.In the Colorado River Basin, the enlargement and conversion of a natural lake into a reservoir rarely, if ever, will increase biological productivity and benefit fishing or related recreation. Raising the level of the lake obviously is for the purpose of using the additional water, which means that a considerable fluctuation will take the place of the former stability. The additional volume of water will not bring a corresponding increase in productivity. Only the addition of permanent, nonfluctuating, shallow water can do this. The prospect of such a loss of original productivity and recreational value, without any corresponding recreational gain, is illustrated by a plan, subsequently abandoned, to dam beautiful San Cristobal Lake on the upper reaches of Lake Fork of the Gunnison River.
Reservoirs in arid regions.More often in the Colorado River Basin the creation of a lake will introduce recreational activities and be of distinct value to the district. Many of the reservoir sites are in arid regions where water is a welcome sight and, though some of the reservoirs may not be ideal for recreational use, they will still have some recreational value. Frequently the water area will provide an opportunity for a number of new enterprises such as cabin camps, boat equipment, supply and storage, fishing supplies, and refreshment establishments.
The construction of a large reservoir on a desert stream profoundly alters the immediate environment, and often improves it greatly for fish and some other kinds of wildlife. This is particularly true if the stream above the lake is heavily silt-laden. Water entering the lake loses its velocity sufficiently to deposit this silt. Productivity of the resulting clear water in a warm environment of high light intensity may be increased a thousand fold, depending upon the degree of water-level fluctuation.
The improvement in the environment resulting from reservoir construction on a desert stream is not limited to the lake itself, and may be even greater in a section of the stream below the lake. The Colorado River, for a 50-mile stretch below Lake Mead, affords an excellent example of this kind of improvement. Water discharged from the lower levels of this deep reservoir ranges approximately between 54° and 61°F.,  whereas temperatures for the undammed Colorado River at Yuma range between 40° and 90°F., with the normal variations perhaps 10°F. less extensive than this.  Violent churning of the water as it passes into the open air from the turbine outlet of the dam renews the high oxygen content. The silt-free water flows for 50 miles or more over clean gravel shoals that are covered by an almost continuous blanket of green algae. 
Growth of such bottom plants in the intense light is rapid, and is uninterrupted because the absence of a severe winter climate permits water temperatures to remain fairly constant throughout the year. The result is that fish-food organisms also multiply enormously and continually. Trout, which normally could not exist within scores or hundreds of miles of this stretch of river, have been introduced with great success. They grow rapidly and so uniformly that the age determinations on the basis of growth rings on scales have been futile. 
Reservoir construction on desert streams can be of great benefit to recreation when, as in the case of Lake Mead, it substitutes a large, clear lake suitable for warm-water game fish, together with a section of Transition Zone trout stream, for the previous turbulent, silt-laden, and unproductive river waters.
By contrast, Lake Havasu, a municipal water supply reservoir 112 miles below Lake Mead, is far less productive of fish and other aquatic life because of the difference in its construction and functions. Although the reservoir is 42 miles long, the dam raises the water level only 75 feet at maximum, and the discharge occurs at only 50 feet below this extreme high-water line. For this reason, the stream below the dam, though silt free, is but little cooler in summer than the undammed river and therefore is unsuitable for trout, though favorable for warm-water fish. 
Still more in contrast to the deep, relatively stable power reservoirs, are most irrigation reservoirs, wherein the maximum drawdown usually occurs in the middle of the growing season for aquatic plants. The amount of drawdown may depend on year-to-year variations in rainfall and in the resulting storage capacity, and therefore may be less excessive during the wetter years. Nevertheless, the general operational cycle of irrigation reservoirs usually is characterized by far greater extremes of fluctuation than is the case with power reservoirs. Production of plant and animal life, therefore, is far from stable, while in extreme cases, which unfortunately are numerous, water levels drop so far that the reservoirs are biological deserts.
Convenience of access is an important factor in determining the value of an area for recreational use. Reservoirs on or near high-standard roads are likely to receive a much larger use than those reached by mountainous, narrow, rough, sandy, or otherwise difficult approach roads. In the case of the smaller reservoirs where the bulk of the use comes from the neighboring communities and farms, travel distance is a prime consideration. If the area is within an hour's drive, the day use may be expected to be of importance; if more than 2 hours' driving distance away, the area probably will not be used extensively unless overnight accommodations are available.
Population of region.In general, the recreational use of a reservoir is in proportion to the population density of the region in which the reservoir is located, but a number of limiting factors enter into the picture, such as the character of the population, climate, nature of site, operation of the reservoir, type and quality of the facilities provided, administration, and maintenance.
It has been found that a larger percentage of the urban than the rural population may be expected to use the reservoirs for recreation; also that reservoirs located in the recognized vacation regions as, for example, the winter vacation region of southern Arizona and the summer vacation region of Colorado, will have a greater use due to the number of people in the region primarily seeking recreation.
The topography of the reservoir area determines to a considerable degree the amount of recreational use that can be made of the area. Reservoirs situated in open valleys usually offer more and better opportunities than those located in steep-sided canyons, where access to the water is difficult and sites for recreational facilities are limited. On the other hand, a very flat slope is also objectionable because of the wide shift of the water line between high and low water levels, frequently leaving unsightly mud flats exposed between the usable land above high water and the lake. The ideal situation is where the top of the dam is determined by a flattening of the topography so that there are gentle slopes adaptable to recreational development near both the high and low water lines.
Sandy soil along the shore line and the presence of trees and shrubs for shade and stabilization of the soil are other assets. Freedom from mosquitoes and other annoying insects is an important consideration in selecting sites for camping, swimming, and other activities.
Outstanding land forms, colored rock, interesting plant growth, or historic and scientific features in the vicinity of a reservoir tend to increase the attendance at the area. Especially are visitors from a distance attracted to the area by the publicity given these special features.
Although the recreational use of a reservoir will depend to a large extent upon the quality of fishing that can be maintained, the adaptability of the reservoir area to a diversity of recreational activities will increase its use and value. As with fishing, however, the quality and adequacy of the recreational facilities will have much to do with the amount of use a reservoir area will receive.
Among the requisites for a successful swimming area are: A comfortable surface underfoot, sandy and free from mud, rocks, and weeds, both above and below the shore line; water temperature about 70°F. or above; protected shallow areas for small children and nonswimmers; and deep-water diving areas. Camping areas are more popular when some privacy is offered between camp sites, which means the presence of adequate plant growth for screening. Parking spaces convenient to the boat docks, beach, and picnic areas contribute to the success of the development.