Page 102


Page 102
Colorado River problem
Is Part Of,8
Full text
SMITH ON THE COLORADO RIVER PROBLEM 407 at the present time for flood protection and for insuring a late-summer irrigation supply is moderate in amount, say, 4,000,000 acre-ft. exclusive of silt storage. He does not agree as to the importance of locating the reservoir for flood protection as far down stream as practicable, and considers that the author's argument against the Boulder Project should not be taken as conclusive until more is known about alternative projects in the Lower Basin. Why begin at Topock? The Mohave Valley is the hottest place known, at least so far as the U. S. Weather Bureau reports show, and the evaporation rate must be excessive; the potential power at that site is unequal to the immediate demand, and the foundations have not been drilled. It is likely that bed-rock will be found considerably deeper than at the Topock railroad bridge, where the maximum depth is not known, except that it is in excess of 100 ft. The Bridge Engineer of the Santa Fe Railway System has stated that "the east pier supporting the main cantilever and anchor arms goes down very close to 100 ft. and is founded on a bed of cemented volcanic boulders." It is not reassuring to contemplate laying the foundations at a depth of more than 100 ft. and at the same time contending with the untamed river in a canyon only 240 ft. wide. The writer believes that no attempt should be made to construct this or the Boulder Dam until considerable river control is secured farther up stream at sites where the bed-rock is shallow. The Topock Reservoir, too, would accumulate silt rapidly, especially on the higher levels, and the storage capacity would be reduced seriously in 20 years unless the Boulder Dam is built in the meantime. Of course, it would be a great advantage to the Imperial Valley to have the river desilted, but, likewise, it would be advantageous to those who operate turbine water-wheels in the Canyon Section. Mindful of the need of quick action in the interests of the Imperial Valley, it seems most untimely for the Engineering Profession to propose a new location for the first dam, and to ask for another year of delay, to permit of determining the foundations at this new site. The first dam should be at the Dewey site on the Grand River, built by the Federal Government as a regulating dam for flood control. Plans for such a dam were detailed by the U. S. Reclamation Service several years ago. With a height of 215 ft. it would create a storage capacity of 2,300 000 acre-ft. The depth to bed-rock is only 44 ft. below the surface. The rock is hard, Carboniferous sandstone, suitable for building operations. The site is above the silt-gathering area and, consequently, the life of the reservoir will be long. Studies of the effect of this proposed reservoir on flood flows showed that in 1914, a year of exceptionally high June floods, the crests at the mouth of the Grand River would have been reduced from 64,000 to 23,000 sec-ft. The regulation of the river effected by the Dewey Dam would give greater security in the construction of other dams farther down stream, and would double the power capacity at Diamond Creek and at other power sites above the mouth of the Virgin River. The site is below all irrigable areas on the Grand River, so that its use for river regulation would not interfere with the irrigation development of the Upper Basin. Its cost is estimated at $11,000,000.

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