WSP03032 - Laserfiche WebLink

Home Browse Search

w 00 tv Ul . The eventual filling of the lake basins with sediment initiated the ances- tral southward drainage from basin to basin. During this period the . Green River was still contained in the Wyoming Basin. The main stem of the Colorado in the Rocky Mountains also initially . drained westward into the ancient lakes. The original course was differ- ent than the present one. Uplifts checked the Colorado River and drainage was intermittent during the Miocene time. . About the end of the Miocene time (ca. 10 million years ago) the mountain barriers to flow were overcome and the present drainage pattern . was established on the main stem. During the Miocene and Pliocene times, the lower Colorado River Region (Basin and Range province) was periodically an estuary of the . Gulf of California. This area received th e Colorado River drainage. The annual discharge of the Colorado River, about lO-l5 million acre-feet, originates almost entirely from the southern and Middle Rocky . Mountains. Negligible runoff is attributed to regions of less than l5 inches annual precipitation. This runoff pattern is considered to have . held throughout the river's history. The runoff from the total basin averages less than 1 inch. The sediment load, however, is one of the highest in the United States. Erosion above Grand Canyon, as estimated . by the sediment load, is about 6.5 inches per l, 000 years. This erosion is attributed mainly to the Cretaceous marine shales which were es sen- tially continuous across the Colorado Plateau when it was uplifted. These . 6 .