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<br />10 WATER AND CHOICE IN THE COLORADO BASIN <br /> <br />many of the tributaries in the lower basin. On August 1, 1905, following a <br />dry year during which a temporary cut, without a headgate, had been made <br />from the Colorado River to the Imperial Valley Canal, the Colorado diverted <br />itself into the canal and flowed, almost unchecked, into the Imperial Valley <br />and the Salton basin-whose lowest point is 273.5 feet below sea level-for <br />more than 18 months, forming the Salton Sea. It thus"repeated, with the ac- <br />cidental help of man, an act it had performed unaided many times in its <br />dynamic history. Again in 1909 the river broke westward but was forced <br />back into its channel by quick erection of levees. <br /> <br />A WIDE RANGE OF ECOLOGICAL ZONES <br /> <br />The Colorado River system gathers its strength from the snowpack of the <br />Rocky Mountains, which in winter extends far below the white cowls of the <br />peaks into the spruce, fir, imd pine forests of the lower slopes and valleys. <br />Winters in the high country are long and biting; the summers short and <br />warm. The growing season is short; Steamboat Springs, Colorado, has only <br />about two frost-free months per year. <br />Below the confluence of the Green River, its principal tributary, the <br />Colorado flows in the shadows of canyons incised into stepped plateaus of <br />almost naked rock. In the plateau country, the winters are cold and the <br />summers are hot. The sparse sporadic rainfall nourishes some of the larger <br />plants, mainly sagebrush, pinon pine, and juniper. <br />Below Bridge Canyon the river leaves the plateaus to flow through one of <br />the hottest, driest regions of the United States, where cacti, mesquite, grease- <br />wood, creosote bush, and yucca mottle the arid surface. Winters here are <br />mild, and summers are hot. The growing season is long; Yuma may have two <br />or three consecutive years without frost. Because of the extreme aridity and <br />the long growing season, the annual water consumption per irrigated acre is <br />high (but the yield is also high; some of the land produces three crops a <br />year). <br /> <br />THE GRAND CANYON: A RENOWNED NATURAL WONDER <br /> <br />The Colorado basin contains many natural wonders, but its best-known phys- <br />ical feature by far is the Grand Canyon. More than 200 miles long, the <br />canyon ranges in depth from 3,500 to 6,000 feet and in width at the top <br />