TECHNICAL BULLETIN NO. dl ^^*t^^^^^/^^^& February, 1928 UNITED STATES DEPARTMENT OF AGRICULTURE WASHINGTON, D. C. SILT IN THE COLORADO RIVER AND ITS RELATION TO IRRIGATION By SAMUEL FORTIER, Senior Irrigation Engineerj and HARRY F. BLANEY, Associate Irrigation Engineer, Division of Agricultural Engineering, Bureau of Public Roads CONTENTS Page Introduction 1 The silt problem of the lower basin 29 Summary and general conclusions 2 Silt investigations in Imperial Valley 28 Colorado River Basin 5 Transportation of silt _ 42 Character of silt 8 Desilting processes - 53 Silt-sampling equipment _. H Appendix - 63 Silt in tributary streams - 13 Literature cited 92 Silt in the lower Colorado River .-- 22 Other publications relating to the silt problem. 94 INTRODUCTION While silt is the creator of much of the agricultural wealth of the lower Colorado River Basin, it is also the greatest menace to irriga- tion development and water control. When irrigation water con- taining silt is applied to fields, the main portion of the silt is deposited near the upper end. From time to time the farmer is compelled to move the deposited silt to lower portions of the field in order to keep the land surface below the level of the irrigation ditch. It is estimated that the annual expense to the farmers of Imperial Valley on account of silt averages $2 per acre. During the next decade vast sums of money are likely to be expended in building structures to control and utilize the waters of the Colorado River, and in the design and location of these structures the silt problem will receive much attention. This is a preliminary report which does not undertake to offer a €omplete solution of the silt problem. The data it presents, while basic, do not remove the need for further extensive and thorough investigations of the sources of silt throughout the entire basin, the lessening of soil erosion, the devising of more efficient desilting faciHties, and the impounding of silt behind permanent dams. Many of the data on which this bulletin is based were obtained cooperatively by the Division of Agricultural Engineering of the 82560—28 1 1 2 TECHNICAL BULLETIN 67, U. S. DEPT. OF AGRICULTURE Bureau of Public Roads/ the California State Department of Public Works, and the Imperial irrigation district.^ They have been supple- mented by the results of investigations carried on independently by the Bureau of Reclamation, the Geological Survey, the Imperial irrigation district, the Division of Agricultural Engineering, and J. E. Peck. Because the interests of the investigating agencies were limited to the lower section of the Colorado River most of the work was done there, but since much of the silt carried into the river's lower reaches is developed in the middle and upper sections, the whole stream has been studied in the preparation of this report. A strong inducement to enlarge its scope developed as the study progressed. At first, main dependence for a solution of the problem of disposing of the silt carried by the river was placed in the design and construction of suitable structures at the diversion points and in the channels, as well as in the design and installation of equipment that would cheaply and effectively remove silt from artificial water-* ways, but the results of the experiments soon suggested that the most economical method was to retain the silt behind masonry dams in the main river and its tributaries. Accordingly, this report has a twofold purpose, one being to aid the irrigators of the lower basin in the better control of silt, and the other to pave the wa}^ for a more complete control of silt by means of storage reservoirs in the middle and upper sections of the Colorado River system. SUMMARY AND GENERAL CONCLUSIONS The Colorado River Basin with its varied physical features is nearly as large as the State of Texas. Those portions which have an annual precipitation of 20 inches or enough to maintain tree growth, are at elevations from 6,000 to 11,000 feet where the soil is too rough and rocky for cultivation, and much of the lower area is too arid or otherwise unfit for profitable farming without irrigation. These limitations, coupled with the cost of providing irrigation water, are likely to confine the reclaimed area to about 7,000,000 acres— less than 5 per cent of the total area. The lower basin, however, possesses exceptional agricultural advantages on account of its mild winter climate, long growing season, and resultant diversity of soil products, as well as exceptional natural facilities for the complete control of the river system by means of impounding reservoirs and the utilization of the water supply for irrigation, power, and other purposes. The economic remedial measures feasible of application to the con- trol of silt in the Colorado River are as follows: (1) The storage of silt in a large reservoir located near the lower end of the canyon section, supplemented by the storage of silt in smaller reservoirs located on tributary streams; (2) the forming of settling basins and 1 The irrigation work of the United States Départirent of Agriculture was originally conducted under the supervision of the Office of Experiir.ent Stations and designated as " irrigation investigations. " Later, under a reorganization of the depart:rent, this and other agricultural engineering activities were grouped in a division of agricultural engineering and made a part of the Bureau of Public Roads. 2 From 1907 to April, 1925, the cooperative investigations were under the general supervision of Samuel Fortier with Clarence E. Tait in direct charge up to the ti: ne of his death in April, 1923. Since April, 1925, W. W. McLaughlin, associate chief of the Division of Agricultural Engineering, has supervised the work. Mr. Tait was assisted at various times by Harry F. Blaney, F. D. Bowlus, H. M. Lukens, and F. J. Veihmeyer. SILT IN COLORADO RIVER IN RELATION TO IRRIGATION Ô the installation of desilting structures at or near the intakes of diverting canals; and (3) the exercise of efficient control over the growth and maintenance of native grasses and other vegetable cov- ering with the twofold object of providing more forage when needed for domestic animals and lessening the injurious effects of soil erosion and silt formation. As discussed in this report, the silt transported by the Colorado River consists of finely pulverized rock with a variable proportion of organic matter. Its color and character vary more or less with the watershed and formation from which it is derived. Normally, the specific gravity of this silt is 2.65, but the weight per imit of volume varies within wide limits. After the river emerges from the canyon section and flows on flatter grades, the heavier silt is deposited or transported as bed silt. The suspended silt transported into the lower basin of the Colorado River is fine in texture, and its particles are of fairly uniform size, fully 50 per cent passing a standard sieve of 200 meshes to the inch. The aggregate quantity of suspended silt in the main tributaries when supplemented by the estimated aggregate quantity in the smaller tributaries, falls far short of equaling the normal load of suspended and bed silt in the Colorado River at Yuma, Ariz., indi- cating the presence of bed silt in the tributaries and the formation of silt in the canyon section by the action of water and wind. The finer silt, or that which passes a 200-mesh sieve, may be transported long distances in both natural and artificial channels if the mean velocity of the current exceeds two-thirds of a foot per second, with a fair uniformity of silt content throughout any vertical section, although there is a tendency for the heavier particles to approach or reach the bed. Thus, any velocity that is practical for an irrigation canal will carry in suspension most of the finer silt of the Colorado River. While great quantities of silt are removed annually from the Im- perial Valley canals by mechanical means it is mainly bed silt, the quantity of suspended silt deposited being a small portion of the total quantity carried in suspension. Usually the suspended silt entering the main canal of Imperial Valley is transported throughout the system. The water delivered to the Irrigators retains most of this silt content, which is deposited in farm laterals or on the irrigated fields. The estimated average annual cost of silt disposal and con- trol in its various forms in Imperial Valley canals is about $1,000,000. By properly designed settling basins, sluiceways, and desilting structures at the intakes of diversion canals it is possible to rid the water of half its suspended silt and most of the bed silt. Formulas held to be appUcable to the tran^ortation of silt in the channels of foreign countries, particularly those of India, do not seem to apply to the water channels of the lower basin of the Colorado River. Determinations of silt content of river water are usually made on a weight basis, which is more practical for research work than a volume basis. ^^Percentage of silt by weight^' is equivalent to the grams of dry silt contained in 100 grams of water, and is derived by weighing the water, then the dry silt, and taking the proportion of the latter to the former. It is believed that the dry sediment in 1 4 TECHNICAL BULLETIN 67, U. S. DEPT. OF AGRICULTURE cubic foot of suspended silt as carried by the Colorado River below Laguna Dam would weigh, on an average, about 62.5 pounds. If this estimate is correct the percentage of suspended silt by weight equals the percentage by volume.