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0296 ~ I I I I I I I I I I I ' I I I I I I II I i and probably represent shallow-lake deposits. The lenses of silt, sand, and gravel were deposited by braided streams as they aggraded their channels. The materials in the Arkansas River valley generally are coarser than those in the tributary valleys and contain fewer clay lenses. The thickness of the alluvium ranges from less than a foot to more than 200 feet in Prowers County. Alluvium is present in the Arkansas River valley both as Pleistocene and recent terrace deposits and as recent flood-plain deposits. Terrace deposits form the major portion of the alluvium in the Arkansas River valley and its tributaries. In the area covered by this study, the terraces are present throughout the Arkansas river valley on both sides of the river and in all the major tributary valleys. Dune sand covers a significant part of the area included in this study. It consists predominantly of very fine to medium sand and includes some coarse sand, but it also contains some silt and clay. The thickness of the dune-sand deposits ranges from less than 1 foot to more than 50 feet; where the dunes are actively migrating the thickness may differ considerably in a few years. HYDROGEOLOGY The alluvium contains the major available supply of ground water in the area covered by this study. Throughout the Arkansas River valley and its tributary valleys, these deposits form an almost continuous unconfined aquifer that is in hydraulic connection with the Arkansas River. Because of its high permeability, the alluvium yields large quantities of water to wells in many parts ofthe area. Most of the irrigation wells obtain their entire yield from the alluvium. Supplemented by water supplied through trans-mountain diversions, the river and alluvial aquifer supply all of the water used for irrigation and a significant portion of the domestic supply for the area. In general the areas of dune-sand deposits are good infi1tration areas for recharge to the underlying alluvial material. However, the few wells in dune-sand areas yield only small quantities of water suitable for only domestic or stock wells. In areas where small saturated thickness of dune sand is underlain by impervious material, a few wells have been drilled through the sand into the impervious material, thus providing a small reservoir for the accumulation of water. HYDROLOGY In the Arkansas River valley, surface water and ground water are two components of one hydraulic system. The valley-fill aquifer is recharged by precipitation, applied irrigation water, and leakage from canals and reservoirs. Recharge to the aquifer from irrigated land is from 45 to 50 percent of the applied irrigation water and precipitation. Ground water withdrawals lower the water table and tend to reduce flow towards the river and, locally, under heavy pumpage, may temporarily lower the water table sufficiently to reverse the ground water gradient and cause water to flow from the stream to the aquifer. The surface water supply has been augmented by diversions to the Arkansas River drainage basin from the Colorado River basin. Ground water return flows that augment the flow