AZ vs. CA:21 California Exhibits, Vol. XXI; Nos. 4501-4597

page 12

-12- from 300 to 500 miles in width, and comprising an area of about 250,000 square miles. Much of this immense area is arid, the mean annual precipitation being Sj4 inches. The northern portion of the Colorado River Basin is an extensive mountainous and plateau region, with an average elevation of 4000 to 8000 feet, and having many snow-clad peaks ranging in elevation from 10,000 to 14,000 feet. Over much of this portion of its watershed the mean annual precipitation is 15 to 20 inches, and it is from this region the river derives the greater portion of its flow. The maximum flow of the Colorado is from 70,000 to 110,000 second-feet and occurs usually in April, May, June or July, as the result of the melting of the snow on the upper portions of its basin. The minimum flow is from 2,500 to 3,000 second-feet, and occurs in September or October, or during the winter months. The mean annual flow is about 10,500 to 11,000 second-feet. The water of the Colorado River carries at all times a large amount of sediment, which gives it a dark muddy appearance. This silt constitutes on an average one percent by weight of the entire flow of the stream, and at times has for short periods reached as high as 3%. - Lower Colorado River Valley - After emerging from the magnificent canyons which form its middle course and thru which it has a very steep gradient, averaging 6 to 7 feet per mile, the Colorado River traverses in its lower course several extensive low-land valleys. Throughout these valleys its gradient is but 1^2 feet per mile and as the current slackens,