RECHARGE CHANGES ALONG RIVERS CROSSING THE HIGH PLAINS AQUIFER

Before the onset of high-volume, consumptive pumping of ground water, rivers crossing and originating within the extent of the High Plains aquifer gained flow downstream from ground-water discharge. Declines in ground-water levels from pumping have now substantially decreased the discharge to most of the rivers. Water-level declines in the aquifer in southwest Kansas have become large enough that ground water no longer usually discharges to a substantial portion of the Arkansas River. A large amount of river flow crossing the western extent of the High Plains aquifer now recharges the alluvial aquifer and then leaks to the underlying High Plains aquifer. During 1995-2000, annual recharge from the river channel in southwest Kansas exceeded 100,000 acre-ft (1.2 x 10⁸ m³). This recharge equates to an average of approximately one ft/yr (0.3 m) as leakage from under the extent of the alluvial aquifer to the High Plains aquifer. An estimated additional 16,000 acre-ft (2 x 10⁷ m³) annually recharged the aquifer in southwest Kansas under canals and land irrigated with diverted river water in 1995-2000. During years of low flow from Colorado and below average precipitation in Kansas, such as in 2002, a substantial stretch of the river becomes dry. The dry sections extended farther to the east into part of the High Plains aquifer in south-central Kansas during the summer of 2002. Although no channel recharge is present in the dry sections, leakage continues from the alluvium to the High Plains aquifer along a substantial stretch of the river. Lower water levels in the alluvial aquifer following dry periods allow greater capture of high river flows from Colorado. Long-term flow decreases in rivers (such as the Cimarron and Beaver rivers) originating within the High Plains aquifer extent, which have resulted from ground-water declines, can cause increases in recharge during substantial precipitation events along upstream stretches that no longer have perennial flow. Projection of future water-level declines in the High Plains aquifer based on continued present and possible additional water use suggest that selected sections of the South Platte and Canadian rivers might reverse from gaining to losing just as has the Arkansas River.