COLORADO RIVER. The Colorado river and its upper tributaries rise in the mountains of Wyoming, Colorado and Utah, where precipitation, especially in the form of snow, is heavy. Including the Green river, the Colorado river is about 1,7oom. long. Its drainage basin covers 244,000sq.m. or . of the area of the United States, and includes parts of seven States— Wyoming, Colorado, Utah, New Mexico, Nevada, Arizona and California. The river forms for 17m. the boundary between Ari zona and Mexico and then flows 8om. through the Republic of Mexico to the Gulf of California. Its discharge varies from about 3,00o second-feet at low water to a flood discharge of more than 20o,000cu.ft. per second. Its water supply, with storage, is esti mated to be equivalent to a uniform flow of 19,300 second-feet at Lees Ferry, 22,600 second-feet at the lower end of Black Canyon, and 21,100 second-feet at Parker. When development in the upper basin is completed and the canyon section and the lower river are developed these quantities will be reduced. With an average annual flow at Yuma of 17,000,000 acre-feet at the present time and an average silt content of 0.62% by volume, the annual load of silt brought to the delta region by the Colorado and Gila aver ages 105,0oo acre-feet, or 170,000,000 cubic yards.
The region traversed by the Colorado and its tributaries is for many reasons of intense interest to the people of the United States. Here was the home of that forgotten people of which there is almost no record except the hieroglyphics on the rocks, the ruins of their irrigation systems, and the cliff dwellings by which they are most widely known; in this region were Spanish missions whose history extends back nearly to the days of Balboa and Cor tez; here is the Grand Canyon, whose grandeur cannot be de scribed; and here are the greatest known natural bridges, so remote and inaccessible that they have only recently been discovered.
For more than I,000m. along its course the Colorado has cut for itself a deep, narrow gorge or canyon, but at some points where lateral streams join it the canyon is broken and these nar row transverse valleys divide it into a series of canyons. Virgin, Kanab, Paria, Escalante, Fremont, San Rafael, Price and Du chesne rivers on the west, and the Little Colorado, San Juan, Grand, White and Yampa on the east have also cut out for them selves narrow, winding gorges or deep canyons. Each river enter ing these has cut another canyon ; each lateral creek has cut a canyon ; and each brook runs in a canyon ; so that much of the upper part of the basin of the Colorado is traversed by a laby rinth of these deep gorges. The longest unbroken canyon through which the Colorado runs is that known as the Grand Canyon be tween the mouth of the Paria and the Grand Wash.
The Grand Canyon is 278m. long and at one place is 13m wide and nearly 6,000ft. deep. All the scenic features of this canyon are on a grand scale. Low plateaux, dry and treeless, stretch back from the brink of the canyon. In some places rock is composed of richly coloured and variegated marls, and here the surface is a bed of loose, disintegrated material through which one walks as though in a bed of ashes. In other places the rock is a soft sandstone, the disintegration of which has left broad stretches of drifting sand, white, golden and vermilion. Where this sandstone is a conglomerate, a paving of pebbles has been left—a mosaic of many colours, polished by the drifting sands, glistening in the sunlight.
On the California side of the lower portion of the river in the United States a vast desert, which has been known as the Colorado desert and more recently as the Salton basin, stretches north westward from the head of the Gulf of California, a distance of 15o miles. At one time in the geological history of this country, the Gulf of California extended a long distance farther to the north-west, above the point where the Colorado now enters it, but this stream brought its mud from the mountains and hills above and bore it into the gulf, across which it gradually erected a vast dam until the waters on the north were separated from those on the south. Then the Colorado cut a channel into the lower gulf. The upper waters, being cut off from the sea, gradually evaporated. The bottom of this ancient upper gulf has come to be known as the Salton sink. It is now land about 278ft. below the level of the sea. On the Arizona side of the river there are similar desert plains, but these are interrupted by mountains.
In the year 1905 there occurred, about 3m. below the Califor nia-Mexico line, a break which diverted all the waters of the Colo rado into the Salton sink and created the Salton sea, with a maxi mum depth of 76ft., a length of 5om. and a width of ion'. to i5m., a total water area of 445 square- miles. The break threatened inundation of the entire Imperial valley and permanent blocking of the Southern Pacific route. Because of this latter danger the Southern Pacific company stopped the crevasse and completed, in 1907, a line of protective levees, the whole involving a deal of engineering skill and an outlay of $2,000,000.
The Colorado problem is international. Under existing treaties neither the United States nor Mexico can take action along the common boundary line which might impede navigation in the Colorado river. The language of the treaties does not, however, appear to prevent the taking of water for navigation purposes from the Rio Grande in the United States above the point where it forms the boundary line between the two countries.
Colorado river is one of the remarkable rivers of the world in its value for irrigation and water-power. It combines, in proper sequence for complete use, a large quantity of water, great con centrations of fall, reservoir sites for the control of flow, sites for power plants, and several million acres of irrigable land below the stretch where power may be developed. Over 5,000,000 prime h.p. can be developed. The flood menace is a serious one; the silt brought down annually to Yuma produces dangerous meandering in the leveed sections in the delta and causes the river to seek new outlets. These threats necessitate large expenditures for revetment and the steady raising of levees to prevent further dis asters such as occurred in 1905.
For the purposes of controlling floods and regulating the flow of the lower Colorado river and for interception of silt, providing for storage and delivery of water for irrigation purposes and for the generation of power, Congress has provided for the construc tion of a dam in the Colorado river at Black canyon or Boulder canyon. As the project entailed an adjustment of the water rights claimed by Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming, a commission comprised of repre sentatives of each of these States and of the United States pre pared, in 1922, a plan for the distribution of the waters, giving 7,5oo,000ac.ft. annually to the upper States (Colorado, New Mex ico, Utah and Wyoming), and 8,5oo,000 to the lower States. A disagreement between Arizona and California delayed ratifica tion of this compact until 1928 when all but Arizona ratified by agreeing to an amendment that the ratification of any six of the States would make it effective. Congress followed by passing the Boulder Canyon Project Act on Dec. 21, 1928 and President Hoover issued a proclamation the following June authorizing con struction. Construction was not formally begun until Sept. 17, 193o, but the rapid completion of four huge diversion tunnels, each 56ft. in diameter, during the next three years, providing a dry bed on which to construct the dam at the boulder, and the erection of a modern city to house 600 workers high above the waters of the projected dam, and the construction of 53 miles of railway and 4o miles of improved highway completed the early stages of the project and attracted national attention to that great engineering feat. The acceleration of public works projects under the Roosevelt administration speeded construction and by 1935 such progress was made that the water was already backed up sixty-five miles behind the world's greatest dam, starting the man-made lake that will eventually extend 115 miles and have an average width of two miles. The hydro-electric plant will dis tribute 1,835,000 horse power and an area of 2,1oo,000 acres will be irrigated. The income will gradually cancel the entire cost of construction, towards which Congress had appropriated $165, 000,000 in 1928 and in the course of fifty years accumulate a surplus calculated at an additional $165,000,000. The dam it self, 73oft. in height and i,000ft. along the crest of its brim, is the largest in the world although several others boast a greater length.
Its services to the Rocky Mountain States will open a new field for colonization provided with an adequate supply of hydro electric power and irrigation.