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Gilcrest/LaSalle Pilot Project Hydrogeologic Characterization Report recharged at locations from which the water will return to the river during times of high surface water demand. This allows well operators to pump groundwater out of priority without injuring senior surface water rights holders downstream. The locations of recharge ponds and other recharge structures, ditches, reservoirs, and the quantity of water recharged are important for interpreting groundwater level and flow conditions. A map showing reservoirs, major irrigation ditches, ponds and other recharge structures in the Study Area vicinity is included as Appendix G. STUDY AREA GEOLOGIC CONCEPTUAL MODEL Figure 3 shows the surficial geology based on Smith (1964) and Colton (1978) and modified to include surficial deposits relevant to this study. Colton's 1:100,000 scale map is the most detailed published surficial geologic map encompassing the Study Area. Subsurface interpretation was performed by creating a database from geologic descriptions in DWR water well permit files. Borehole geologic log locations used for lithologic characterization are shown in Figure 4 and the Microsoft Access (MS Access) database is included in Appendix B. Geologic cross-sections through the Study Area are shown in Figure 5. Geologic material descriptions in the many logs obtained from the water well completion records vary considerably due to inconsistency among logging personnel; mostly water well drillers. To standardize and simplify interpretation, lithologies were grouped into the following general types after Lindsey (2005) and modified for this study: topsoil, gravel, sand, sand and silt, silt and clay, and bedrock. The database is limited to information from the well intervals through the alluvium; deeper bedrock descriptions are not included. The relevant surficial geologic units mapped in the Study Area (Figure 3) are described by Colton (1978) and Smith (1964) and modified for this hydrogeologic characterization as described below. Minor isolated deposits unlikely to be in hydrologic communication with the aquifer are not described in detail below. It should be noted that Colton (1978) and Smith (1964) mapped much larger areas and descriptions may include unit thicknesses and localized characteristics observed outside the Study Area. Surface Topography The Study Area topography is characterized by broad fluvial valley gently sloping upward away from the South Platte River to low hills along the southwest valley flank and a low ridge separating the South Platte Valley from Beebe Draw (Figure 2). Only the alluvial valley surface overlying the aquifer will be discussed here. The highest portion of the alluvial valley in the Study Area is the ridge separating the main South Platte Valley from Beebe Draw which has an elevation of approximately 4, 950 feet above mean sea level (ft MSL), and the lowest portion is in the northeast portion at I.