COLORADO RIVER. A large river flowing through the plateau region of the southwestern United States. It is formed in the southwestern part of Utah. by the junction of the Green River front the north and the Grand from the north east, the former rising in southwestern Wyoming and the latter in the north central part of Colo rado. Both of these headstreams receive nu merous tributaries from the well-watered regions of the Rocky Mountains. Relow their junction, the Colorado passes through what is in some respects the most remarkable region on the earth, not only for its natural scenery, but also for the great interest which it possesses for geologists, as it gives on a grand scale the clearest exem plifications of the action of erosive forces in shaping the contour of the land. In the Eocene epoch the whole region of Arizona, Utah, and Nevada was subjected to a vast upheaval, and what was formerly the bottom of the ocean was raised to a height of more than 10,000 feet above sea-level. This region, consisting chiefly of hori zontal strata of the Paleozoic and Mesozoic sys tems, was subjected to extensive denudation by wind and water, and again to successive upheav als accompanied by volcanic action. In the northwestern part the strata were faulted into huge blocks, running north and south, giving the present shape to the mountains of Nevada. The result of the uplifting and erosion was the washing away of the weaker and softer strata, especially to the west of the present course of the Colorado, while those rocks that were protected by harder layers were left standing as exten sive plateaus with precipitous escarpments. In some places lavas had been thrust up through the strata by volcanic action, and these localities are now marked by the isolated mesas so char acteristic of the country. Since this region is almost or quite rainless below an elevation of 8000 feet, denudation proceeds slowly except along the river-courses, where chasms or ea lions are cut deeply into the rock foundations.
Such is the case with the Colorado and all the tributaries from its headwaters to the great escarpment called the Grand Wash, on the west ern boundary of Arizona.
The largest and deepest of these is the famous Grand Carion. where the Colorado cuts throhgb the Kaihab and Unikaret plateaus, from 7000 to 9000 feet high, in the northern part of Arizona. The canon is five to six miles wide at the top and 5000 to 6000 feet deep, falling in several successive escarpments, indicating pauses in the upheaval of the plateau. In the middle is the narrow and gloomy carton proper, with a sheer precipitous depth of 2000 to 3000 feet, at the bottom of which rushes the river. The length of the Grand Canon is over 200 miles. After the river emerges from the caftan it turns abruptly south, and, forming the western bound ary of Arizona, it flows tri•ough a low desert region, receiving almost no tributaries, and di minishing in volume by evaporation and absorp tion. Leaving United States territory near its mouth, it empties into the Gulf of California. The total length of the river is about 900 miles, and with the Green, 2000 miles. 11. is navigable for light steamers for several hundred miles from its mouth, but navigation is much impeded by rocks and sand-bars, as well as by the ever changing volume of its water and the of its bed. The river was discovered in I540by Fernando Alar•6n, and the perilous descent through the canon was first made by James White in 1807. Valuable additions to geological science have resulted from expeditions into the canon, conducted by the United States Geologi cal Survey. See J. W. Powell, Exploration of the Colorado Piro of the West and its Tribu taries: and Dutton, Tertiary history of the Grand ('a iron of the Colorado, Monograph II., United States Geological Surrey, !SS?.