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. l-& ~ o w . .; COLORADO RIVER SALINITY CONTROL PROGRAM (continued) 2. Grand Valley Unit The Grand Valley is located near the western edge of Mesa County in western Colorado. Grand Junction in the aouthern part of the valley is the largest city in Colorado west of the Continental Divide. The valley was carved in the Mancos Shale formation (a high salt bearing marine shale) by the Colorado River and its tributaries. Within the valley the irrigated lands have been developed on recent alluvial plains consisting of broad coalescing alluvial fans and on older and higher alluvial fans, terraces, and mesas. Other lands in this arid setting, where rainfall averages only about 9 inches (230 mm) per year, include stream flood plains, rough broken te~race eBcarpments~ high knobs, and remnants of former mesas. Irrigation in the valley began in 18B2 with the construction of what is now the Grand Valley Canal (Grand Valley Irrigation Company). Other private systems were built during the period between 18B2 and 1908. Con- struction of the last major system, the Grand Valley Project under the Reclamation Service, began in 1908 with the major construction completed in 1926. This project consists of two divisions, Garfield Gravity and Orchard Mesa, on the north and south sides of the river respectively. A total of about 71,000 acres (29 000 ha) are served water by these irrigation entities with approximately 38,000 acres (15 000 ha) under the Federal project. Major crops produced in the valley are corn, small grains, alfalfa, pasture, and various orchard crops. The Grand Valtey is estimated to contribute an average of about 780,000 tons (708 000 t) of salt annually to the Colorado River. Most of these salts are leached from the soil and underlying Mancos Shale by deep perco lat ion and seepage from water del ivery systems, and are washed into the river by ground water inflow. The Mancos shale is a very thick sequence of saline drab gray fissile shale that lies between the underlying Dakota sandstone and the overlying Mesa Verde Formation. The thickness of the shale usually varies between 3,000 feet (900 m) and 5,000 feet (1500 m). Due to this great thickness and its easy erodibility, the shale fiUs most of the large valleys in western Colorado and eastern Utah. Many white patches of salt and alkali are visible on the nonirrigated surfaces and some patches are also visible on the irrigated lands where drainage is poor. The salts present in the Mancos shale are mostly calcium sulfate with smaller amounts of sodium chloride, sodium sulfate, and magnesium sulfate. Calcium sulfate (gypsum) is commonly found in crystal form in open joints and fractures of the shale. Due to the compactness of the clay and silt particles making up the shale, the formation is not considered water bearing at depth. However, the weathered zone near the surface does transmit water along joints, fractures, and open bedding plains. This zone is the area from which percolating water, often originating from irrigat ion of croplands, may dissolve salts present in the shale. In addition, most of the soil forming the irrigated lands have been derived from Mancos shale. As a result, the soils are also a source of salt. 67