National Park Service A Survey of the Recreational Resources of the Colorado River Basin

LOCATION

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., [1] 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. [2] 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. [3]

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. [4]

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. [5]

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.

NATURE OF SITE

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.

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 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 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 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.

In general, a drawdown of over 18 inches or two feet would have a deleterious effect on many food plants for ducks, even though cattails, giant bullrush and some others of little or no value would still do all right. On the other hand, a rather slow drawdown of ten or twelve feet, provided it did not have too much effect on the area of the reservoir . . . might be good for fish by minimizing weed growth. [7]

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." [8]

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.

Fluctuation, especially if it is at all rapid, may have an extremely deleterious effect on the bottom fauna, such as the Tubifex worms, and larvae of Chironomus and other midges. These are of very great importance as fish food . . . The bugs that burrow in the surface of the mud (and these are probably of greatest importance to the fish) have very little motility . . . The fish are concentrated, since most of them stay in the reservoir, while the bottom fauna dies off in considerable part, and the plankton, which has little control over its general movements, drains out with the water or else readjusts its total mass to the new size of the lake within a few days. This leaves the fish in a pretty pickle. [9]

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.

Strawberry Reservoir, in Utah, . . . was built on a tributary of the Duchesne River to store water for irrigation and happened to cover a very productive meadowlike area. The production of trout in that area was phenomenal, but the oxygen demands of the abundant organic material in the basin was sufficient to suffocate the trout while under the ice of mid-winter. The drawdown during the irrigation season was so great that the trout were forced to live in the oxygenless zone near the lake bottom. [10]

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.

WATER TEMPERATURES

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 [11] 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 [12] with 70°F. close to the upper safe limit, and 80°F. too high if sustained for any length of time. [13] 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. [14] The general life zone requirements of the two types of fish already have been discussed in Chapter I.

FERTILITY OF THE RESERVOIR

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. [15] 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. [16]

ENDNOTES

1 Moffett, loc. cit., p. 77.

2 Dill, op. cit., p. 130.

3 Moffett, loc. cit., p. 79.

4 Moffett, loc. cit., p. 83.

5 Dill, op. cit., p. 112.

6 Clarke, 1939, p. 29.

7 Bond, 1945, letter.

8 Bond, lit. cit.

9 Bond, 1945, letter.

10 Moffett, James W., letter, 1/15/46.

11 Hasler, 1938, p. 94.

12 Needham, 1938, p. 313.

13 Wales. 1939, p. 303.

14 Dill, 1944, p. 130.

15 Moffett, 1942, p. 80.

16 Moffett, James W., lit. cit.