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. along. the lower reaches of the Colorado. .Over a large portion of the w ao N W lower valleys 50 to 60 inches is an overall average for evaporation. . Physiography. The drainage basin of the Colorado River is about 250,000 square miles. The Colorado River is l,440 miles long and its . drainage basin extends into five physiographic provinces. lts headwaters originate in the Southern and Middle Rocky Mountains and the river flows through the Wyoming Basin, The Colorado Plateau, and the Basin and . Range province in its lower reaches. More than half of the drainage ba- sin is contained in the Colorado Plateau. This plateau was uplifted and tilted northeast in geologic time, forming a temporary block of the river. . The Colorado River, where it leaves the Colorado Plateau, formed the Grand Canyon as it cut across the rim of the plateau. . For that portion of the river which rises in the Southern Rocky Mountains much of the area is higher than lO, 000 feet in altitude with peaks reaching more than l4, 000 feet. Much of the Colorado Plateau is . between 5,000 and 7,000 feet in altitude and the river crosses this pro- vince in deep canyons where it exits in the Grand Canyon. In the Basin . and Range province the river is below 2,000 feet in altitude. In the Wyoming Basin, the relief along the Green River, the chief tributary of the Colorado, is relatively minor and in places the valley . bottoms are wide. ln contrast, where the river flows through the middle and southern Rocky Mountains the valleys are steep sided, generally narrow am 2,000 to 3,000 feet deep where they cross the mountain . 4 .