Laserfiche WebLink
<br />2353 <br />".'- <br /> <br />Th'e irrigation system is operated on a demand basis by the <br />Bureau of Indian Affairs. The main canal and laterals are operated <br />relatively full and excess water is released through wasteways to the <br />drainage system. The system is operated for approximately 335 days <br />of the year, and shut down 30 days for maintenance. In 1974, meas- <br />ured diversions from the river were 648,766 acre-feet, and in 1976, <br />586,494 acre-feet. The reduction occurred while the irrigated acreage <br />increased, probably reflecting recent efforts to operate the distribu- <br />tion system more efficiently and higher than average rainfall. <br />Irrigated agriculture thrives on the Reservation and a variety of <br />field crops and vegetables is grown. Predominant field crops are <br />alfalfa, cotton, and wheat. Major vegetable crops are lettuce, onions, <br />and melons. Multiple cropping occurs on approximately 11 percent of <br />the irrigated land. Border and furrow irrigation are the predominant <br />methods used, and application is usually controlled to prevent tail- <br />water flow from fields. Average onfarm irrigation efficiency on flood <br />plain lands was estimated to be ~6 percent in 1974. [2] <br />The Reservation is served by a system of open drains that <br />collect subsurface return flows and discharge them to the Colorado <br />River. The principal drains generally flow in a southerly direction <br />for 8 to 15 miles and then bend westerly to meet the river. Measured <br />drainage from the Reservation was 352,885 acre-feet in 1974. <br />Geology <br />The soils and substrata of the flood plain are a mixture of <br />alluvial gravel, sand, silt, and clay deposited by the Colorado River. <br />The most typical lithology of these deposits is medium-grained sand <br />with gravel lenses and layers. Vertical thickness of the unconfined <br />alluvium in the project area is generally 130 feet. <br />The unconfined alluvium is underlain by the Bouse formation, <br />which is composed of (from top to bottom) a unit of interbedded clay, <br />silt and sand, and a basal limestone, and was formed in a marine to <br />brackish-water environment. The uppermost contact of the Bouse <br />formation is marked by a layer of clay which, on the basis of data <br />from three Geological Survey test wells, appears to be from 15 to 35 <br />feet thick, and to lie at a depth of about 130 feet under most of the <br />flow plain. The depth appears to increase at the southern end of the <br />8 <br />