Several major river systems cross the High Plains aquifer from west to east. These river systems include the Platte, Republican, Arkansas, Cimarron, and Canadian Rivers. The alluvial-aquifer system associated with each river is also an important local water resource. The High Plains aquifer is in hydraulic connection (water can move from the High Plains aquifer to the alluvial sediments associated with the rivers and visa versa) with the major river systems crossing the aquifer (Weeks and others, 1988). During low-flow periods, water in the rivers is almost entirely derived from ground-water discharge. Land use within the Study Unit primarily is agriculture and rangeland (figure 4).

Regional variability of water-level changes in the High Plains aquifer results from large regional differences in climate, soils, land use, and ground-water withdrawals for irrigation. Substantial pumping of the High Plains aquifer for irrigation since the 1940's has resulted in water-level declines of nearly 150 feet in some parts of the aquifer.