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ranges from zero to approximately 240 feet. The regions of greatest saturated thickness can be <br />seen along the center of the paleochannel from Fort Morgan to Julesburg as indicated by the red <br />color fill. Areas that show a color fill of black represent unsaturated areas and are generally <br />confined to the edges of the alluvial extent, or between tributaries. <br />The saturated thickness in the areas of the Big Sandy Designated Basin and the southern end of <br />the Kiowa Bijou Designated Basin are less accurate than other areas of saturated thickness. <br />Saturated thickness is based on a subtraction of two grids and the accuracy is dependent on the <br />accuracy of the source grids. The source data for these two basins, is of lower resolution (100- <br />feet contour interval compared to 20-feet contour interval elsewhere) and, consequently, the <br />saturated thickness estimated from the source data is less reliable. However, on a regional scale, <br />the map presented provides a sufficient understanding of the geometry of saturated thickness <br />for the South Platte Alluvium Region as a whole. <br />3.5 Cross sections <br />Figure 11 presents the locations of six cross sections developed for the South Platte Alluvium <br />Region. The cross sections provide another means to visualize the alluvial aquifer configuration. <br />From one cross section to the next, changes in elevation and lateral aquifer configuration from <br />Henderson to the Nebraska state line are evident. Figures 12 through 17 show the cross sections <br />with the elevation of the land surface, bottom of alluvium, and the water table surface. The <br />bottom of alluvium and groundwater level data shown were based on the grids developed as <br />discussed in Section 1.2. Land surface elevations were extracted from the 2001 USGS 30-meter <br />NED. All cross sections are presented with a vertical exaggeration of a 23:1 vertical to horizontal <br />ratio. This provides sufficient exaggeration to view the incised paleochannels, yet still reflect the <br />broadness of the alluvium. <br />The configuration of the alluvial valley at each cross section provides insight into its creation <br />and development. Each of the cross sections shows an incised river channel carved into the <br />bedrock surface by the ancestral South Platte River. Down-cutting likely occurred near the end <br />of successive glacial periods in Colorado's more recent geologic history. As glaciers near the <br />Continental Divide receded, the increased melt water runoff had sufficient force to erode the <br />bedrock surface and create relatively deep and narrow channels. As the last glacial melting <br />event ended in the Quaternary Period, the flow in the ancestral South Platte River declined, as <br />did the energy to erode into the bedrock. Decreasing river flow caused suspended sediment to <br />settle out, gradually filling the former river channel up to the present day elevation. Cross <br />sections A-A' to F-F' demonstrate this geologic history. Beginning at cross section A-A', the <br />paleochannel is shallow and broad but moving downstream the cross sections reveal <br />paleochannels are more incised with steeper banks. Some of the cross sections reveal more than <br />one paloechannel indicating the streams have meandered over time or the cross section may <br />extend into a tributary paleochannel. The following paragraphs describe each cross section in <br />more detail. <br />Cross section A-A', shown in Figure 12, is located near Brighton at the Henderson gaging <br />station. The cross section lacks the V-shaped geometry that can be seen in the other cross <br />sections. Along the western edge the bedrock surface is steeply dipping for 0.5 mile and then <br />gradually rises to the east for 4.5 miles and generally parallels ground surface. Near the eastern <br />SPDSS Phase 3 Task 42.3 TM -Final 13 <br />11/30/2006 <br />