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<br />0176 <br /> <br />Return Flows <br /> <br />~ <br /> <br />Irrigation-drain water flows to the Arkansas River in tributary streams, <br />in drainage ditches, and through the alluvial aquifer. The locations of <br />numerous irrigation drains in the study area in Colorado were documented by <br />Cain (1985). Some of these drains are extensive, well-maintained systems, <br />whereas others are simply small, unmaintained ditches or channels. Minimal <br />documentation exists concerning irrigation return flows to the Arkansas River <br />in Kansas, but types of irrigation drainage similar to those in Colorado may <br />be present. Minimal information is available to quantify the volume of return <br />flow in either State. <br /> <br />Much of the streamflow in the Arkansas River downstream from La Junta <br />may be irrigation return flow during parts of most years; therefore, any water <br />that is diverted from this reach of the river may be affected by irrigation <br />drainage (Cain, 1985). Also, the entire alluvial aquifer may be affected by <br />irrigation drainage, either through recharge from the river or by direct <br />percolation of irrigation water through soils. <br /> <br />Ground Water <br /> <br />The principal source of ground water in the Arkansas River basin in <br />Colorado is the alluvial aquifer. This aquifer ranges from almost 0 to about <br />300 ft in thickness and from 3 to 6 mi in width (Taylor and Luckey, 1974). <br />Saturated thickness in the alluvial deposits ranges from almost 0 to 100 ft, <br />and depth to water ranges from almost 0 to 75 ft below land surface. <br /> <br />West of the Bear Creek fault (fig. 3), the alluvium fills a trough that <br />is eroded into shale, limestone, and sandstone bedrock. The bedrock is <br />relatively impermeable and is a barrier to downward water movement. The <br />alluvial aquifer in this area is recharged primarily by irrigation drainage, <br />seepage from the river, and regional ground-water inflow in the area of <br />Coolidge (Luckey, 1975; Barker and others, 1983). Within the alluvial <br />aquifer, water flows to the southeast under a hydraulic gradient of about <br />8 ft/mi. Water is discharged from the aquifer by well pumpage, seepage to the <br />river, evapotranspiration from shallow water-table areas, and outflow to the <br />Bear Creek fault zone. <br /> <br />East of the Bear Creek fault, the alluvial aquifer overlies unconsolidated <br />material of the Ogallala Formation to which it is hydraulically connected <br />(Dunlap and others, 1985). This aquifer system is recharged by inflow from <br />the part of the alluvial aquifer west of the fault, by ground-water inflow <br />from deposits of Pleistocene age north of the Arkansas River valley, by <br />infiltration of precipitation and irrigation water, and by seepage from the <br />Arkansas River and irrigation canals. Saturated thickness is about 50 ft near <br />the fault zone but decreases to almost zero between Lakin and Garden City. <br />This decrease is due to leakage from the alluvial aquifer to the underlying <br />Ogallala Formation, which has been pumped for irrigation for many years <br />(Dunlap and others, 1985). <br /> <br />17 <br />