Page 113


Page 113
Colorado River problem
Is Part Of,8
Full text
418 LANE ON THE COLORADO RIVER PROBLEM examined for the purpose of retaining a relatively small storage, from which no primary power would be obtainable. The conflicting views as to whether the supply of water is greater or less than required by the available irrigable lands, are evidently traceable to varying judgments concerning lands suitable for reclamation. Field observations made by the writer on the head-waters and in the intermediate valleys of various tributaries justify the opinion that all the feasible sites at the higher levels should be utilized as rapidly as they can be financed. This is particularly true of the Gila River in Arizona, where the direct benefits to irrigation and the incidental reduction of silt burdens arid flood menace for the lower delta will be of immense value. About twenty years ago field data were collected and designs made for the Gila under the direction of Arthur P. Davis, Past-President, Am. Soc. C. E., and others. The use of this material should prevent the delays and controversies over the Colorado River problem which at present seem to be impending. E. W. Lane,* Assoc. M. Am. Soc. c. E. (by letter) .—The effect of the deposition of silt on the storage capacity of the proposed basins in the Colorado River has been given careful consideration, but has any study been made of the effect of discharging into the river below the flood-control dam of from 40,000 to 80,000 cu. ft. per sec. of water from which the silt has been removed? It is obvious that the water will pick up and carry away large quantities of silt, and although there is probably little, if any, precedent for determining the quantity thus removed, it is easy to show that this may be important. The silt now passing the Boulder Dam site is estimated to be 80,000 acre-ft. per year. Neglecting the effect of the redistribution of flow and diversions, if it is assumed that before traveling the 459 miles to the Gulf of California, the stream has picked up as much silt as it is capable of carrying, it would remove practically 80,000 acre-ft. of silt per year. Assuming a river bed 1,500 ft. wide, this quantity would be equivalent to an average depth of nearly 1 ft. per year. It would seem more reasonable to expect it to pick up this in the first 100 miles, in which case, the average lowering would be about 4.5 ft. per year; at the upper end it might be twice this, or more. As the silt percentage curve (Fig. 10) given by the author shows that the silt content was nearly the maximum at a discharge of 40,000 cu. ft. per sec., the redistribution of flow might even increase the silt removal more than 80,000 acre-ft. per year. The quantity of silt removed from the river bed below the dam will be influenced by a number of factors. As the river bed is cut down, the tributary streams will cut down their beds also, bringing into the main stream large quantities of silt which will reduce the rate of lowering of the main stream bed. The fewer and smaller the tributary streams, therefore, the more rapid will be the lowering of the main stream bed. If bed-rock lies close to the river bottom or heavy gravel deposits exist, such conditions will retard or limit the erosion in their vicinity, but this will only tend to increase it in other parts of the stream. Diversions of water, of * Asst. Engr., Dayton Morgan Eng. Co., Pueblo, Colo.

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