64 



SCIENCE 



[Vol. LVI, No. 1438 



order to expose the minimum possible area for 

 evaporation losses and prevent waste of water 

 thereby. 



Sixth, lowest possible cost giving due weight 

 to other considerations. 



From all data available to date, the Black 

 or Boulder Canyon sites, only I8I/2 miles apart 

 by river, seem to fill the above requirements 

 more nearly than any other site or sites which 

 have been found. 



The proper size of the reservoir is a matter 

 not yet fully determined, the factors entering 

 into this feature being irrigation storage, flood 

 detention storage, silt storage and power de- 

 velopment. 



The maximum capacity so far considered is 

 31,400,000 acre feet, including 5,000,000 acre 

 feet of silt storage, 11,400,000 acre feet for 

 irrigation storage, 10,000,000 acre feet for 

 power development and 5,000,000 acre feet for 

 flood control. Other capacities considered have 

 varied from 8,000,000 acre feet up to the max- 

 imum. Recent estimates include a reservoir, 

 with dam located in Black Canyon to store 

 23,500,000 acre feet, which would leave a 200 

 foot head available for the development of 

 power between this reservoir and the so-called 

 Diamond Springs power project. 



If built for flood control purposes alone, a 

 reservoir with capacities of 5,000,000 and 

 8,000,000 acre feet is estimated to be suffi- 

 cient to hold discharges at Ymna due to up- 

 stream floods down to 75,000 and 50,000 second 

 feet respectively except in rare instances. In 

 connection with a reservoir for irrigation pur- 

 poses, the addition of 5,000,000 acre feet stor- 

 age for flood control alone is expected to pro- 

 vide the desired reduction in floods. 



In order to visualize the size of the reser- 

 voir to be impounded it may be said that in 

 case of the largest reservoir considered, the 

 lake when full will have a surface of 240 

 square miles and will be about 90 miles long, 

 with a 40 mile arm extending up the Virgin 

 River. If all the water in the reservoir were 

 to he discharged through a conduit 10 feet 

 in diameter at the rate of 10 feet per second 

 it would requii'e six years to empty the reser- 

 voir provided no water entered it during that 

 time and neglecting evaporation losses. 



A great many preliminary designs and esti- 

 mates have been prepared of various types of 

 dam for both Boulder and Black Canyons in 

 connection with various plans for the develop- 

 ment of power. The conclusion reached is 

 that a dam of the gravity type, built on a 

 curved jDlan, is the most conservative in de- 

 sign and best suited to a dam of such unpre- 

 cedented height. 



The dam in Boulder Canyon proposed for 

 the largest reservoir considered would be 600 

 feet high above the present low water surface 

 in the river and 750 feet from the lowest point 

 in the foundation to the highest point on the 

 dam. This would be more than twice the 

 height of Arrowrock Dam, which is 348% 

 feet high and which, at present, is the highest 

 dam in the world. The length on top would 

 be approximately 1,250 feet and at the bottom 

 the dam would be nearly as long up and down 

 stream as it is high. The amount of concrete 

 required to build the dam and appurtenant 

 structures with a reasonable allowance for 

 contingencies is roughly estimated at 4,700,000 

 cubic 3'ards. This would be four times the 

 concrete contained in Arrowrock and Elephant 

 Butte dams combined, and if used to build a 

 column 100 feet in diameter, the height of the 

 column would be three miles and would weigh 

 91/2 million tons. 



The preliminary estimate of the cost of this 

 dam is about $58,500,000. About 700,000 firm 

 horsepower could be generated with a power 

 plant costing $28,000,000 and the transmission 

 line will cost about $20,500,000 more, or a 

 total of about $107,000,000. 



In connection with the dam, it is proposed 

 to provide outlet capacity for the discharge 

 of 25,000 second feet of water for irrigation 

 use and a spillway capacity of 200,000 second 

 feet with water surface in the reservoir 10 

 feet below the top of the dam. Under condi- 

 tions of extreme floods the latter will be 

 increased to 300,000 second feet at the time 

 the water surface in the reservoir reaches the 

 top of the dam. As a part of the spillway it 

 is proposed to provide an opening at the 

 bottom of the flood storage with no provision 

 for closure. By this means water would start 

 disehai-ging as soon as the reservoir is full to 



