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Show TABLE 1.-Area of the Colorado River Basin by States 1 Basins of interior drainage included within the limits of the Colorado Basin on accompanying maps are not part of this estimate. 8 Not estimated. Source: Corps of Engineers. The headwaters of the Colorado River system rise in these mountains and flow toward the interior of the basin. Because of the plateau's elevation many streams have cut deep canyons, and the broad sweep of the plateau surface is scored by the tremendous trenches of these streams. The southern part of the basin is a low desert plain out of which rise scattered low mountain ranges. The climate of the basin is as varied as the to- pography. The mountains and high plateaus are cold with relatively heavy precipitation; the south- ern deserts are very hot and dry. The temperature has varied from an extreme of 54° below zero Fahr- enheit in the upper basin to 127° above in the desert. Annual averages, however, are about 40° in the upper part of the basin and over 70° near the mouth of the river. The summers in the northern section of the basin are warm to cool and of moderate length. In the south they are long and hot. Likewise, the winters are long and cold in the north, but mild or even warm in the southern section-so warm that year- round cropping and production of semitropical fruits are possible. With the exception of the mountain areas, the entire Colorado Basin is arid. In the northern part of the basin the cool temperatures and precipitation of 10 to 15 inches produce a vegetation of sagebrush and grass over most of the plateau areas. Aridity increases steadily toward the south and vegetation changes to desert shrub, and finally, in parts of the lower basin, the most barren conditions in the United States prevail. In some of these southern areas annual precipitation is less than 5 inches and is accompanied by a high annual temperature. Differences in humidity are greater than differences in precipitation because the higher average temper- ature in the south greatly increases the rate of evaporation and thus makes the scanty rainfall even less effective. The mountains of the basin, on the other hand, are generally humid. Annual precipitation reaches 30 inches in all of the mountain areas and many mountain sections receive as much as 40 to 50 or more inches. These mountain areas furnish a dis- proportionately large share of the runoff of the basin in comparison with their size. Winter precipitation usually comes from general storms which sweep in from the north Pacific. The rains are normally rather gentle so that much which falls below the snow line percolates into the ground. These winter storms often cover many thousands of square miles over the basin. Similar storms at other periods are rare, the summer storms being violent and of short duration. Because of high tempera- tures, most of the rain reaching the ground is lost by evaporation before it gets into the streams, although occasionally a summer storm covers a large area and may result in appreciable runoff. The snows which fall at the higher elevations are the chief source of stream flow. They accumulate as much as 15 feet during the winter months from November to April. At high elevations, winds may sweep the snow from open lands into protected canyons or old cirques where it may form an ice pack that lasts throughout the summer season. Ex- cept for these protected places, most of the snow melts by July. 357 Per-c_nt Per-Approxi- + + i cent State mate J£\t £ basin Square United States: miles Arizona.................* 103, 000 *90 42 California................ 6,000 4 2.5 Colorado................ 39,000 38 16 New Mexico............. »23, 000 119 9. 5 Nevada.................. 12,000 11 5 Utah.................... 40,000 47 16 Wyoming................ 19,000 19 8 Total in the United States. 242, 000 ...... 99 Mexico: Sonora and Baja California..................... 3,000 (2) 1 Total in Mexico........ 3, 000 ............ Grand total.................. 245,000 ...... 100 |