THE COLORADO RIVER BASIN
From Gannet Peak in the Wind River Mountains of Wyoming and the 14,000-foot mountain peaks in Colorado to the Salton Sea in southern California, 248 feet below sea level, stretches the vast region of forests, deserts, plains, mountains, canyons, and plateaus drained by the Colorado River.
The outline of the Colorado River Basin as it follows the crests of adjacent mountain masses is irregular, but its general shape may be compared with the ear of a horse. The lower end of the basin in southern California, Arizona, and New Mexico, corresponding to the base of the ear, is broad, being some 500 miles wide at one point, and is bounded for the most part by low, desert ranges. Northward, in Utah and Colorado, the basin becomes progressively narrower, edged by loftier mountain ranges, until at the extreme northern tip of the ear, 830 miles from the Mexican border, it is reduced to a width of 25 miles as it culminates in the magnificent Wind River Range of western Wyoming.
The Colorado River Basin not only is ringed with mountains but contains within its boundaries many extensive ranges and lofty plateaus. In the north, these interior ranges run in an east-west, or else a northwest-southeast direction, forming a series of more or less parallel mountain barriers and high plateaus across the basin. Between these barriers lie great valley systems, also running more or less east and west. Only the mighty Green and Colorado Rivers have cut deep gorges in a southerly direction through the barriers to unite all the streams into one drainage. Toward the south, the interior ranges pivot gradually from the northwest-southeast direction, until in southern Arizona, Nevada, and New Mexico they run nearly north and south. The physiography of the basin can be most readily described in terms of these various mountain barriers and their intervening valleys.
Farthest to the north lies the spacious but bleak valley of the southward-flowing Upper Green River in Wyoming, with the elevated and still more barren Red Desert to the east. These areas have long, cold winters and short, dry summers. They extend south into northern Utah and Colorado where they encounter the first of the mountain barriers. This barrier is formed by the Uinta Mountains and Yampa Plateau as they extend eastward toward, but do not quite meet, the Danforth Hills and White River Plateauwhich in turn are westward extensions from the main Rocky Mountain Divide. Winters are still longer and colder than in the valleys, with a heavy snowfall. Summers are cool, with frequent afternoon thunderstorms.
Beyond this first barrier to the south lies the broad, east-west valley of the Duchesne River, elevated and bleak, but farmed, and its counterpart on the opposite side of the Green River, the wilder but similar valley of the White River. These valleys have long cold winters and short dry summers like the Upper Green River Valley. They are bounded on the south by the second mountain barrier, comprising the Tavaputs Plateaus which, with the Roan Plateau, form an 8,000- to 9,000-foot wall all the way across the basin from the Wasatch Mountains on the west to the White River Plateau of the Rockies on the east. Only the Green River has been able to cut a cleft through this wall to join with the stream systems still farther south. The climate of this mountain barrier resembles that of the first, being characterized by deep, long-lasting winter snows and cool summers with afternoon thunderstorms.
About 50 miles south of the TavaputsRoan Plateau barrier, the southward-flowing Green River joins the Colorado River which, with its great tributary, the Gunnison, flows southwest from the heart of the Rockies through broad sagebrush valleys to this junction point. The Colorado River beyond this junction plunges into a wild and fantastically eroded land of winding gorges and sandstone mesas whose vast expanses are punctuated at irregular intervals by the isolated, steeply upthrust masses of the Henry, Abajo, and Navajo Mountains. With the exception of the mountain summits, which are cool and moist, the greater portion of the area receives but little snowfall in winter and is characterized by a long, warm summer season. Average temperatures are higher than those of valleys to the north, in conformity with the decrease in latitude, but lower than those of deserts to the south. The annual precipitation ranges from about 6 to 14 inches, with the greatest amount coming from thunderstorms during July and August.
This desolate but spectacularly scenic sandstone area is referred to in this report as the Canyon Lands of southeastern Utah. Into it from the east, through a deep meandering gorge, flows the turbid San Juan River which, with its tributaries, drains a far-reaching area, including the La Plata Mountains, the San Juan Mountains and other lesser ranges on the Continental Divide, the Chuska Mountains, and enormous areas of mesa land in the interior of the basin. From the Wasatch and Aquarius Plateaus on the west come other, but lesser, silt-laden streams, all carving the deep, winding canyons which have given this country its name.
The Canyon Lands terminate, approximately, at a third, intermittent barrier formed by the Paria Plateau, Black Mesa, and the Chuska Mountain mass. Southwest of this third barrier lies the great valley of the Little Colorado River and the adjacent House Rock Valley a semidesert grassland, piñon pine, and juniper country which, in spite of its enormous area, contributes but a meager stream flow because of the sparse annual rainfall and snowfall. Winters are mild and summer temperatures about the same as in the Canyon Lands.
South and west of the Little Colorado River, the last great highland barrier extends in a vast curve across the Colorado River Basin from the southern boundary of Utah through Arizona to western New Mexico. This barrier is formed by the Grand Wash Cliffs, the Coconino and Kaibab Plateaus, the San Francisco group of volcanic peaks which rise above 12,000 feet, the long but narrow Mogollon Plateau averaging above 7,000 feet, and the 11,000-foot crests of the White Mountains. Heavy snowfall, long winters, and cool summers with afternoon thunderstorms characterize the climate of most of these highlands.
West and south of this last barrier lies the great southern desert of Nevada, Arizona, California, and Mexico. Rainfall is scanty and the summer season long and intensely hot. Winters are mild. Many low mountains rise from this desert. The most important of these ranges are the Hualpai, Harquahala, Santa Maria, Bradshaw, Pinal, Santa Catalina, Rincon, Galiuro, Pinaleno, Santa Rita, and Chiricahua Mountains. They comprise a series of cool islands rising out of the surrounding desert heat rather than a climatic or physiographic barrier.
The life zone map (Pl. 12, in pocket) summarizes graphically the natural features of the Colorado River Basin. It indicates the climates of the basin and their characteristic plants and animals. In view of the close relation between temperatures and altitudes, to be described later, the map also shows enough of the topography to furnish a rough three-dimensional presentation of the basin, the extent of the many great plateaus, the island-like distribution of many of the southern mountain ranges, and the deep penetration of the Rocky Mountains by numerous long, winding valleys.
The life zone map indicates the nature and distribution of the basin's various recreational regions. The Lower Sonoran Zone is principally a winter recreation region, where the desert scenery and sunshine are ideal for photography, swimming, camping, hiking, horseback riding, and motoring, and where some hunting and fishing may be enjoyed. The Upper Sonoran Zone, in valley and plains lands, provides recreational opportunities similar to those of the Lower Sonoran Zone, but is usable in summer when the latter region is too hot for comfort. The northern limit of the Upper Sonoran Zone is generally open sagebrush country not suitable for winter recreation.
The Transition Zone of the foothills and lower mountains probably attracts more vacationists than any other because it is the principal retreat of those seeking to escape from the summer heat of the lowlands. The location of this zone at the lower mountain levels usually renders it easily accessible at all seasons, while the coolness which accompanies its ample precipitation makes it the principal zone of winter sports. In the Colorado River Basin, water is most plentiful in the Transition Zone, so that it includes most of the best natural fishing areas, and for similar reasons the finest forests. With this abundance of vegetation are associated most of the big game animals, affording maximum opportunities for hunting and wildlife photography. In fact, nearly every form of outdoor recreation may be found in the Transition Zone and many kinds reach their highest development there.
The Boreal Zone is the high mountain recreational region. In the northern half of the basin it is visited nearly as frequently as the Transition Zone because it extends to fairly low levels. However, in the southern half of the basin it is, as a rule, sufficiently difficult to reach that only those who can appreciate its wilderness character are likely to penetrate it very deeply or permanently. High mountain recreational developments usually are restricted to the general vicinity of the relatively few roads that penetrate Boreal regions, and such developments are further restricted in their operations by the extreme shortness of the summer season. Camping, hiking, fishing, horseback riding, winter sports, photography, and the enjoyment of wildlife and rare flowers reach their culmination for many persons in this remote, and, to many, most beautiful and inspiring of all recreation regions.
Because the basin embraces latitudes from Mexico almost to Yellowstone National Park, and altitudes ranging from 248 feet below sea level to 14,431 feet above sea level, all of the life zones of the United States are present except the Tropical Life Zone of southern Florida. These life zones are as follows:
Boreal Zone.In detailed studies of small areas this often is subdivided into three zones Alpine, Hudsonian, Canadian but these finer distinctions are needless here. This zone is characterized by the cool, moist climate of Canada and the northern coniferous forest belt of the United States. Winter snows are heavy and long-lasting. Total annual precipitation is from 23 to 60 inches or more. On the other hand, at the lower levels of the Boreal Zone in the arid Upper Green River Valley of Wyoming, annual precipitation may be as low as 15 inches in some localities.  The annual mean temperatures are in the neighborhood of 45° to 27°F. or less. Narrow tongues of the Boreal Zone extend far south of the main area along various high mountain crests such as the Rockies. With the increase in average temperature toward the Mexican border, the tongues give way to a series of scattered islands as the zone becomes restricted to the tops of the loftiest peaks.
In the Colorado River Basin, the most reliable and conspicuous plant indicators of the Boreal Zone are the lodgepole pine, limber pine, whitebark pine, white pine, foxtail pine, white fir, alpine fir, Engelmann spruce, blue spruce, dwarf juniper, aspen, red elderberry, dogwood, and the blueberries. Extensive timberline grasslands and scattered high mountain meadows also occur in this zone.
Some of the more interesting or conspicuous animals and birds that characterize the Boreal Zone are the pine squirrel, marmot, cony, snowshoe rabbit, lynx, red fox, marten, wolverine, goshawk, three-toed woodpecker, Rocky Mountain jay or camp robber, clark crow, and crossbill.
Transition Zone.The zone was given this name because it was considered to be essentially an area where the ranges of many northern (Boreal) and southern (Sonoran) plants and animals overlapped. Although this is to some extent true, the Transition Zone possesses a quite distinctive climate and a number of prominent and characteristic species of its own.
In Wyoming, Colorado, and northern Utah, where the Boreal Zone covers all the major mountain masses, the Transition Zone occupies the foothills and higher valleys. Farther south, however, where the Boreal Zone is restricted to projecting tongues and isolated high peaks, the Transition Zone occupies most of the mountain masses, and is replaced in the valleys by a warmer climate. Winter snows are moderate. Total annual precipitation is from 17 to 26 inches, occasionally as much as 32 inches, and sometimes more.  Annual mean temperatures usually are between 40° and 50°F., except in a few Transition areas of extreme cold and aridity such as the Upper Green River Valley at Pinedale and Kendall, Wyo. 
Throughout a large part of the Colorado River Basin, this zone coincides very closely with the open, parklike ponderosa pine forests of the mountains and foothills, but north of the San Juan Mountains, Uncompahgre Plateau, and the Aquarius Plateau, the Transition Zone becomes a partially open region, characterized by sagebrush valleys and grassy slopes, often covered with Gambel oak and other chaparral shrubs. This reduction of forest growth may be due to the fact that precipitation in that region is relatively light even at the higher altitudes, while soil moisture and average relative humidity are further reduced by frequent high winds.  In this northern part of the basin the ponderosa pine gradually is replaced by Douglas-fir, but distribution of the latter within the zone often is restricted to steep slopes, shady areas, or the crests of plateaus above 7,000 feet, leaving the major part of the zone to the sagebrush and chaparral.
Other plant indicators are the narrow-leaved cottonwood, Utah oak, bur oak, bearberry, Rocky Mountain birch, Oregon or holly grape, elderberry, maple, locust, and alder. The sagebrush previously mentioned cannot be used as an exclusive indicator of the Transition Zone because it occurs even more extensively in the next warmer zone and sometimes enters the lower limits of the Boreal Zone. Since hardly any two kinds of plants, or animals, have precisely the same climatic requirements, the location of zone boundaries is based upon the presence of not one but a large number of the appropriate indicators. Obviously, therefore, the establishment of sharp lines of division between zones represents merely a compromise among various arbitrary decisions that it is necessary to make in order to draw up an intelligible small-scale map.
Animal indicators confined to, or particularly characteristic of, the Transition Zone include the tuft-eared squirrels, certain local races of wood rats and pocket gophers, and other small rodents, wild turkey, and band-tailed pigeon.
Upper Sonoran Zone.This and the next zone take their names from the Province of Sonora in Mexico, from which they derive some of their plants and animals. Climatically, the Upper Sonoran Zone is considerably cooler than much of Sonora. It comprises most of the broad lower valley lands of the basin except in southern and western Arizona, southern Nevada, and adjacent California, where it gives way to a still warmer climate and becomes the principal life zone of the foothills, extending even to the summits of the lower desert mountains. Snow is light or absent, and the summers are warm. Total annual rainfall usually is from 10 or 12 to 18 inches, occasionally more, and annual mean temperatures are between 50° and 65° F.
The Upper Sonoran Zone is typified by the seemingly endless foothill and mesa regions, sparsely covered with junipers (3 species) and piñon pines, that comprise vast areas in the Colorado River Basin. The zone is practically coextensive with these trees wherever they occur but is by no means restricted to such areas. It includes also extensive regions formerly covered with nutritious grasses where soil and moisture are too meager for piñons and junipers; also the alkali plains, sagebrush flats, and parched wastelands of the Upper Green River Valley and Red Desert in Wyoming, the sandstone barrens in the Canyon Lands of Utah, and the shortgrass and alkali plains of the Little Colorado River.
Some parts of the Upper Sonoran Zone that have a slightly greater rainfall and higher mean temperature than the piñon-juniper region support an extensive belt of scrub oaks.  Growing conditions seldom are favorable enough to permit these scrub oaks, which comprise several species, to become full-sized trees. Usually they are low, spreading shrubs that form dense thickets covering thousands of acres of foothill country. The Upper Sonoran oak brush is prevalent in the disconnected chain of foothills and low mountains south of and paralleling the Mogollon Plateau in Arizona, and is well exemplified in Kirkland Valley and the hills south of the town of Prescott. It also occurs, though apparently less extensively, in some comparable foothill areas of southeastern Utah and southwestern Colorado, such as the Abajo Mountains. 
Additional plant indicators are the sycamore, boxelder, greasewood, hackberry, cliffrose, silk tassel, rabbitbrush, winterfat, and antelope brush. Mammal indicators include several species of small rodents, shrews, and bats which are not listed here because of the difficulty encountered by the average person in distinguishing these species from quite closely related animals inhabiting adjacent life zones. Such a list would be of practical use only to the technical worker, who can readily obtain such information elsewhere.
Bird indicators also are numerous, but, as in all of the life zones, only those are given this status that are known to nest in the zone in question. After nesting season, most of these birds spread into adjacent life zones, so that here again, a list of breeding birds would be of little help to any but the technical worker. The few birds that are listed as indicators elsewhere in this outline are believed to be year-round residents of the zone concerned. The piñon jay probably comes the closest of any bird to being a year-round resident of the Upper Sonoran Zone, but even this species wanders briefly into other regions.
Lower Sonoran Zone.This, the last and lowest of the Colorado River Basin life zones, is the hot southern desert zone. It extends from Mexico northward into Arizona as far as the highland barrier formed by the Grand Wash Cliffs and the previously described chain of foothills and low mountains that parallel the Mogollon Plateau and White Mountains. It completely surrounds many of the southern desert mountains as an ocean surrounds an island, particularly in the southeastern part of the State, while other mountains of lesser height, including the many low ranges of southwestern Arizona, are completely submerged by it. This zone occupies the Salton Sea region (among other places) of California and extends long fingers into the lower valleys of southern New Mexico. It occupies the desert plains and valleys of southern Nevada and extends a dilute arm for a short distance into the adjacent Virgin River Valley of southwestern Utah. Another dilute extension of the zone threads its way up the Colorado River, deep beneath the surrounding land surface, between the sheer, heat-reflecting walls of the Grand Canyon. It extends briefly at the lower end of the Little Colorado River Valley in the vicinity of Cameron, and in the lower Paria and Wahweap Valleys above Lees Ferry.
Snow is practically absent, and annual precipitation averages from 1-1/22 to 12 or 15 inches. In some parts of the zone there may be no rain at all for 2 or 3 years, but irregularly occurring cloudbursts help to raise the average figure. The summers are long and in many areas intensely hot, and annual mean temperatures are between 51° and 74°F.
The Lower Sonoran Zone is the zone of mesquites, many kinds of cacti, and the creosotebush. The mesquites are too dependent upon subsoil moisture to occur throughout this zone, while some kinds, though not all, of the cacti are sufficiently tolerant of cold to occur well beyond its limits. The creosotebush is the most conspicuous, widespread, and characteristic single plant of the Lower Sonoran Zone, and is almost coextensive with it except that it does not occur in the poorly drained alkaline soils of the desert sinks.
Other Lower Sonoran Zone plant indicators are the screwbean, catsclaw and other acacias, ocotillo, desertthorn, allthorn, paloverde, smoketree, tesota, bluethorn, varnish bush, arrowwood, century plants, and sotol.
Mammals used as indicators by ecologists include the round-tailed ground squirrels and cotton rats, as well as various other rodents whose close relatives occur in adjacent zones. Among birds, the white winged dove, Arizona cardinal, and Arizona pyrrhuloxia are good indicators where they occur, but their range within the zone is not as wide as that of the mammals.
In general, the higher the altitude the cooler the air; roughly, the temperature decreases 3-1/2° F. for each increase of 1,000 feet in elevation above sea level. Stated in another way, an increase of 1,000 feet in altitude is equivalent to a northward shift in latitude of approximately 200 miles. An easily remembered illustration is experienced by anyone who travels over the desert in summer by airplane. On taking off at approximately sea level from a desert airport that is sweltering under a temperature of, say, 110° F. in the shade, a few minutes' climb to 5,000 feet brings a reduction of temperature to 92° F., followed by a further drop to 74° F. if one continues through the upper layers of air to the 10,000-foot level. One can even fly over Death Valley during midsummer in complete comfort in a plane at 11,000 feet, and no doubt this new form of recreational travel will become increasingly important in desert regions and elsewhere.
The lowering of temperature with increasing altitude helps to explain the permanent coolness of mountaintops, and why they constitute islands with a northern climate rising above the surrounding ocean of warmer air. On these island summits cling assemblages of cold-climate plants and animals that have been stranded there since the last Ice Age, when more than one-half of the continent was buried under a vast, unmelting crust of ice and snow. In southern regions these mountaintops usually are choice recreation areas.