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of the Front Range averages about a mile in width and has an irregular surface that consists of <br /> swamps, oxbow lakes, abandoned meander scars, and low, indistinct terraces (Smith et al., <br /> 1964). The overall surface drainage in the region is toward the northeast. Surface topography <br /> consists of many terraces and subtle changes in topographic relief that can make differences in <br /> water table depth over short horizontal distances. The major perennial tributaries of the S. Platte <br /> River in the project area are Clear Creek, Big and Little Dry Creeks, St. Vrain Creek, the Big <br /> Thompson River, Cache la Poudre River, Lone Tree Creek, and Crow Creek. Several <br /> intermittent streams also enter the river below Kersey, including Kiowa, Bijou, Badger, Wildcat, <br /> Beaver, Pawnee and Cedar Creeks. <br /> The alluvial aquifer is in hydraulic communication with the surface water system throughout the <br /> basin, and the extensive development of irrigation, reservoirs, transbasin diversions, and wells <br /> has resulted in gaining conditions for the majority of the river since application of irrigation <br /> water results in deep percolation, and resulting return flows to the river. The maximum thickness <br /> of the alluvial deposits increases in a downstream direction on the mainstem with saturated <br /> thickness of 20 to 40 feet in the upstream region near Denver to more than 200 feet near <br /> Julesburg (Map 4). Well depths in the lower S. Platte River basin alluvium average about 75 feet <br /> below ground surface. The hydraulic characteristics of the aquifer are such that high-capacity <br /> irrigation wells may yield 1,200 to 2,000 gallons per minute. Hydraulic conductivity is the main <br /> physical parameter that governs the rate of groundwater flow, varying considerably within <br /> relatively small areas in the alluvial aquifer. Hydraulic conductivity (K)values in the S. Platte <br /> alluvial aquifer range from approximately 20 to 2,000 feet per day (with a median value near 500 <br /> ft/day) depending on the materials present. Infiltration from precipitation, irrigation, canal <br /> seepage, and pond seepage recharge the alluvial aquifers whereas groundwater tends to discharge <br /> to the main channel of the river. Groundwater discharge to the river channel creates baseflow for <br /> the river. <br /> All groundwater in Water Division 1 that is not either Designated groundwater or Denver Basin <br /> groundwater is presumed to be tributary groundwater, in direct hydraulic connection to the <br /> surface stream system. However, there are a number of water right decrees in Water Division 1, <br /> generally entered from 1910 to 1970 that specifically declare the groundwater to be nontributary. <br /> The almost 500 so-called Coffin Wells in Water District 1 and 3 were decreed as non-tributary <br /> by Judge Coffin in 1953, although today we know they are in the alluvial aquifer and are indeed <br /> tributary to the S. Platte River. <br /> 42 <br />