DEVELOPMENT OF AN ENDORHEIC BASIN IN THE WATERSHED OF THE UPPER ARKANSAS RIVER, USA

Seven natural endorheic basins in the U.S. have catchment areas of about 10,000 km² or more, the largest of which is the Great Basin. However, the Colorado River basin has developed into a nearly closed basin due to water use within the basin if its lowermost part in Mexico is included; its area is greater than that of the Great Basin. Another watershed that has developed into an endorheic basin as a result of water use is the upper Arkansas River basin in Colorado and southwest Kansas; its area places it as the third largest in the U.S. after the Colorado basin and the Great Basin. About 5% of the upper Arkansas basin in Kansas is a natural closed basin. The main area of the endorheic basin has developed from decrease in Arkansas River flow across the plains of eastern Colorado as a result of evapotranspirative consumption of water diverted for irrigated agriculture. A smaller amount of river water is diverted for irrigation in southwest Kansas. The remaining flow in Kansas disappears into the alluvium and then the High Plains aquifer (HPA) as a result of substantial groundwater-level declines from irrigation pumping. Today, nearly 200 km of the river bed in southwest Kansas is dry. Substantial sections of the riverbed started to become dry in the mid-1970s as HPA water levels dropped below water levels in the alluvial aquifer. From July 1976 to July 2014, flows passing through southwest Kansas have occurred less than 18% of the time; no through flow has occurred since June 2001. Arkansas River water entering Kansas is saline, with total dissolved solids and sulfate concentrations typically greater than 3,300 mg/L and 2,000 mg/L, respectively. In addition, the uranium concentration usually exceeds 50 µg/L (maximum contaminant limit for public supplies of drinking water is 30 µg/L). The High Plains aquifer in southwest Kansas is now the ultimate repository of this saline river water at the end of the closed basin. If changing climatic conditions result in less winter snowpack accumulating in the headwaters of the Arkansas River in central Colorado, fewer high-flow events that could result in basin through flow will occur. In addition, continued declines in HPA water levels will make it more difficult for high flows to escape the basin downstream because infiltration will rob more of the high flow.