THE COLORADO RIVER BASIN (continued)
Although latitude and altitude are the major factors influencing the climate of the Colorado River Basin, many interesting and striking modifications of climate by minor influences are visible even to the casual traveler and recreationist, and a knowledge of some of their causes heightens interest in the region.
Direction of slope.South-facing mountainsides are exposed to the warming and drying effects of sunshine to a much greater degree than are north-facing slopes. On steep slopes, the difference often is so extreme that a given climate or life zone, say the Upper Sonoran oak brush, may extend on the south side all the way to the summit, at 7,500 feet, in spite of the low temperature of the surrounding air at this altitude. On the shady, moist, north side of this mountain, on the other hand, the Upper Sonoran Zone may give way near the base of the mountain, at 6,000 feet, to a cool forest of ponderosa pine. One may see just such a contrast on the opposite sides of innumerable hills, buttes, and low mountains throughout northern Arizona and New Mexico and southern Utah.
The warming effect on south-facing slopes is still further increased by the circumstance that an inclined surface facing the sun at a right angle may receive almost one and one-half times as much heat and light as a level surface. The inclined north-facing slopes on the other hand, receive less sunlight than a level surface.
Additional results of this difference are that snowbanks on north-facing slopes linger longer, and soils warm up and dry out more slowly or not at all. Spring flowers come later and birds may commence nesting 2 weeks later than their neighbors on the other side of the mountain. On many mountain peaks in the southern part of the basin, it is only on the shady north slopes that sufficient moisture remains throughout the dry season to permit the existence of a Boreal Zone fir forest. This situation occurs on Sitgreaves, Bill Williams, O'Leary, and the San Francisco Peaks, and in the Santa Catalina Mountains, as well as numerous other places in Arizona.  In general, mountain peaks intercept a greater precipitation than lowlands. The degree of interception is approximately proportional to the altitude of the peaks above the lowlands, the precipitation increasing about 1 inch per 160- to 200-foot rise.   In some places, however, the atmospheric currents often have been so robbed of their moisture by the time they reach the lee side of the mountains that the latter have a dry climate that is out of proportion to their altitude. Blacks Fork in the lee of the Wasatch Range is an example of this. Though at an elevation of 9,500 feet, it has an annual average precipitation of only 21.6 inches. Similarly, in Utah, Monticello receives much more winter and spring precipitation than La Sal at the same altitude, because the former is on the windward side of high mountains, whereas La Sal is on the lee side. 
Moisture.In general it has been established that for an area such as the Colorado River Basin, the northward distribution (or on a mountain, the upward distribution) of plants is determined by the lowering temperatures, while the southward (or downward) distribution is limited by the progressive decrease of moisture.
In many parts of the basin there are striking illustrations of the ability of northern zone plants of the higher mountain levels to survive in the lower zones, provided their moisture requirements are satisfied. Two or three miles southwest of Lyman's Reservoir, near St. Johns, Ariz., a small isolated cinder cone, grassy and otherwise devoid of trees, has on its shady north-facing slope a shallow steeply sloping ravine within the narrow confines of which there stands a slender file of ponderosa pine. Obviously a snowbank lingers in this shaded defile long after other snows are gone, thus providing moisture sufficient for the small colony, which is a mile or more from the main body of the forest and at a lower elevation. Similarly, countless mountain streams through the basin, long after descending from the cool highlands into the warm lowland valleys, are lined with pines and other mountain trees which, having been carried down as seeds, were able to establish themselves along the moist stream banks in the midst of an otherwise alien environment.
Spectacular examples of climate extensions downward into a lower zone may be seen in the Canyon Lands of Utah. There the streams, upon leaving the mountains, frequently vanish into narrow, shadowy gorges of great depth which wind erratically for miles below the rolling surface of the sandstone mesas, on their way to join the canyon of the Colorado. The Transition Zone pines, in following these streams down from the mountain slopes, likewise disappear beneath the surface of the desert, and may be glimpsed only from the brink of one of these gorges or from an airplane. This reversal of the usual altitude relationships is well illustrated in Indian Creek and Montezuma Creek Canyons at the base of the Abajo Mountains, and similar examples occur over great areas of the basin, as on the Black Mesa, and at the edge of the Defiance and Mogollon Plateaus in Arizona. Likewise, the outlet streams of Boulder, Fremont, and other large lakes at the foot of the Wind River Mountains, in the Upper Green River Valley of Wyoming, carry long tongues of Boreal Zone lodgepole pines and aspens down through the treeless, grassy Transition Zone.
Air currents.Cold air tends to flow flown canyons from the high peaks, especially at night, thus further promoting the downward migration of northern plants and animals along the stream banks.
On the other hand, rising currents of warm air, especially those which sweep over mountain slopes from adjacent desert lands, have a reverse effect and carry the climate of the lower levels to a much higher elevation than usual. The Colorado River Basin contains countless examples of this upward shifting of the lower zones adjacent to desert areas.
The Gila Mountains in southeastern Arizona exceed 6,000 feet and under other circumstances would be expected to have a Transition Zone pine forest at the summit. The heat of the San Simon Valley, however, carries the Upper Sonoran Zone to the summit of the range. A similar condition occurs, in varying degrees, in the Little San Bernardino Mountains north of the Salton Sea in California; in the Virgin Mountains of southern Nevada and northeastern Arizona; the Kaiparowits Plateau and San Rafael Swell in Utah; and in extensive foothill regions of the Green River Valley north of the town of Green River, Wyo.
Mountain masses.The larger the mountain mass, the greater the area of cool surface, and the less the likelihood of its being affected by the warming influences of surrounding lowlands. The great frigid mass of the Rocky Mountains, together with such lofty spurs as the San Miguel and Elk Mountains, and the White River Plateau, is able to maintain a Transition Zone climate at lower altitudes than can the isolated Henry and Abajo Mountains in southwestern Utah.
A result of the proximity of mountain masses that is particularly noticeable in desert regions is the greater soil moisture at their bases derived from run-off.
The distribution of fresh-water fish often is closely related to stream and lake temperatures, but because such temperatures may be greatly modified by local factors extraneous to the general climate, such as source and volume of stream flow, proportions of sunlight and shade, and the size and depth of lakes, fish have not been used as life-zone indicators. Nevertheless, in a broad study of an area as large and diversified as the Colorado River Basin, certain useful comparisons may be made between fish and other wildlife regarding the general effect of climate on distribution.
Boreal Zone.The snow-fed, headwater streams and lakes of the high mountains are small, cold, and poorly supplied with fish food. The lack of food results from the prolonged low temperatures which permit a growing season of only a few weeks for aquatic plants and the various small organisms that feed on them and, in turn, are eaten by fish. The low temperatures retard the growth of fish directly by reducing the food supply, and indirectly by slowing up their metabolic rate. In addition to the adverse effects of low temperatures, there is a deficiency of organic matter and of plant nutrients in general in the higher parts of the Boreal Zone. Bare rocks and melting snows that produce water almost as pure as distilled water can provide only meager nourishment for the aquatic plants that comprise the basic link in the fish-food chain. Boreal Zone fish usually remain small no matter how long they live. Mortality during the long, rigorous winters runs as high as 60 percent.  Often, where artificially planted in high mountain lakes, they remain half-starved throughout the season and willingly take any lure, thereby affording some sport even though their size and the condition of their flesh may not always be up to par.
In addition to the native trout, various non-native trout have been planted in the basin, including the rainbow, brown, and eastern brook. For all of them the Boreal Zone comprises a submarginal habitat, and they do better in the warmer, more abundant waters of the Transition Zone.
Transition Zone.The streams here are cold enough for trout requirements, but not so cold as to inhibit their growth. They traverse greater areas of fertile soil and pick up larger amounts of plant nutrients than Boreal Zone streams. Transition Zone streams, which drain extensive, moist mountain masses such as the Wind River, Uinta, and Rocky Mountains, may become large rivers occupying broad, fertile valleys and fringed by dense vegetation furnishing vital supplies of terrestrial insect food in addition to the abundant aquatic sources. The Transition Zone affords by far the finest natural trout fishing, and is exemplified by the Gunnison River in Colorado and its tributaries. Although trout are the most conspicuous and best-known fish of these mountain waters, a few minnows, suckers, and bullheads are present. 
Upper and Lower Sonoran Zones.After the streams have traversed the long, sunny Transition Zone sagebrush valleys of Wyoming, Utah, and Colorado, or have dropped from the limited plateau and mountaintop Transition areas of the States to the south, the water has warmed up beyond the survival point for trout. This occurs at about 7,000 feet,  thereby coinciding approximately with the lower limit of the Transition Zone on land. Below this point the water not only becomes warmer, but also becomes more quiet and thereby loses the high oxygen content required by trout. It often picks up a yellow or chocolate-colored burden of silt which smothers food-producing aquatic plants, insects, and crustaceans, and buries spawning grounds. A few Upper Sonoran Zone streams which are fed by cold springs, such as Oak Creek and Bright Angel Creek in Arizona, are partial exceptions to this rule for a short distance, and provide fine trout fishing.
Trout soon disappear from the Upper Sonoran Zone, but a few more or less silt-tolerant species of warm-water fish are able to survive there. In the lower stretches of the Colorado and Green Rivers, the Colorado "white salmon" occurs in limited numbers. Actually it is an enormous minnow, occasionally attaining a length of 3 feet, and a weight of 30 pounds. Small streams may harbor suckers, top-minnows, and true minnows; the larger streams may contain, in addition, sunfishes, bonytails, perch, and catfishes, provided there is a zone of aquatic shore vegetation in which such fish spawn, and where their young can hide. Such plants also probably furnish some food. The turbid waters of the principal lowland rivers of the Colorado River Basin either fluctuate so greatly with the seasons, or have such a violent scouring action that such streamside and bottom vegetation is usually sparse or lacking.
Artificial reservoirs often completely reverse original stream conditions and may introduce the equivalent of an entirely different "life zone." This will be discussed more fully elsewhere and is mentioned here only to complete the zonal description of the basin. The construction of deep reservoirs like Lake Mead, which discharge large and fairly constant volumes of cold water from which the silt has settled out, has about the same effect as though a section of Transition Zone stream were substituted below the lake for a stretch of the previously barren, hot, and silt-laden river. In this cold stretch of water, trout have been introduced with great success, while in the clear waters of the lake itself, downstream from the silt deposition area, bass thrive, as well as such other introduced species as the bluegill and crappie, and the native warm-water fish previously mentioned. Carp also have been introduced into lower Colorado River waters but are considered undesirable.
Recreational resources are all natural and cultural features which stimulate, encourage, or provide for the physical, mental, and cultural well-being of the individual. The mountains, with rushing sparkling streams shaded by tall pines in which birds sing; the call of a loon across a lonely lake; the overpowering spectacle of the Grand Canyon; Navajos and their sheep at a water hole in Monument Valley; cowboys riding fence or driving in cattle for the spring round-up; the cry of coyotes across the valley; trout and bass in Lake Mead luring the fishermen to the desert; Hoover Dam; the Arizona Biltmore; the log cabin on Grand Mesa; the old mining towns such as Ouray, Silverton, Aspen, and Tombstone; the abandoned mine dumps where you may find some silver; the long purple shadows on the desert pavement; brilliant sunny days and starlit nights; the great open spaces and views without end; silence so complete the hum of a bee sounds like a distant airplane; Meteor Crater, that forceful reminder of the cosmic spaces, and Sunset Crater, equally forceful evidence of the earth's inner heat; Cliff Palace at Mesa Verde; Betatakin; pieces of broken pottery made centuries ago to hold corn; dinosaur tracks in red sandstone; petrified tree trunks; sea shells on a mountaintop; a startled deer leaping across a fragrant sagebrush flat these and many more are the recreational resources of the Colorado River Basin. Nowhere else in the United States do the curious and fascinating land forms, evidences of the building up and wearing down of the earth's surface, stimulate more interest in geology. Nowhere else are the evidences of prehistoric peoples so obvious and numerous. Nowhere else is there scenery so colorful.
Here is the world's greatest canyon, the largest natural bridge, the largest man-made lake (in volume), and the highest dam. Here too, one may enjoy much sunshine and find perfect climates for outdoor recreation the year around. Hunting, fishing, photography, snow sports, boating, swimming, horseback riding, camping, mountain climbing, exploration the entire realm of outdoor recreational activities may be enjoyed. Five national parks and 33 national monuments have been established within the basin to preserve some of the most outstanding natural, scientific, and cultural features. Large sections have been included in national forests, wildlife refuges, and grazing districts, and vast areas have been set aside as Indian reservations. Much of the basin is in public ownership, but this is not surprising when one sees the country and knows that about half the basin has a population of less than 2 persons per square mile, and that the most densely populated county, Maricopa County, in which Phoenix, the largest city, is located, has a population of only 20.2 persons per square mile.
It is only natural in a region so endowed that recreation should become one of the major industries. Agriculture is restricted almost entirely to irrigated sections. Mining, lumbering, and the raising of cattle and sheep first attracted settlers to the basin, but the recreational features are now attracting many more, and as the various sections of the basin become better known and more accessible to the densely populated regions of the United States through improved highways and air transportation, catering to the recreationist should become a major industry. To foster this industry it must be recognized that recreational use of land may, in certain places, be the highest or best use of the land for the general welfare of the people in the basin and, in vast sections of the basin, should be on an equal basis with other uses, such as grazing or production of timber. The great stretches of open range, unobstructed by buildings, fences, transmission lines, and other signs of modern civilization, comprise one of the most important recreational features of the basin. As other sections of the United States become more and more highly developed, this one feature of the Colorado River country, if preserved, will have unusual appeal.