The site of Boulder Dam is in Black Canyon of the Colorado River, approximately 75 miles below the mouth of the Grand Canyon (Fig. 1). At the dam, the river, when at low-water stage, is slightly less than 650 feet above sea level. As the top of the dam is 580 feet higher, the water in the reservoir can rise almost to the 1230-foot contour. The resulting lake will be 115 miles in extreme length, and will have several long, narrow arms in valleys tributary to the Colorado. The width will vary from a few hundred feet in the narrow canyons to a maximum of eight miles in the low country near the mouth of the Virgin River.

As a result of field studies in neighboring areas, the writer realized that this lake would cover geologic features of great interest. Only hasty reconnaissance surveys had been made of most . . .

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