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SPDSS_Task42-3_Phase3_LowerSPlatteAlluviumRegionAquiferConfiguration
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SPDSS_Task42-3_Phase3_LowerSPlatteAlluviumRegionAquiferConfiguration
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Last modified
4/17/2013 9:20:41 AM
Creation date
6/11/2008 10:36:35 AM
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Decision Support Systems
Title
SPDSS Task 42.3 - Lower South Platte Alluvium Region Aquifer Configuration - Phase 3
Description
This Technical Memorandum was undertaken under Task 42.3, and summarizes the compilation, analysis and mapping of existing published aquifer configuration data for the Lower South Platte Alluvium Region.
Decision Support - Doc Type
Task Memorandum
Date
11/30/2006
DSS Category
Groundwater
DSS
South Platte
Basin
South Platte
Contract/PO #
C153953
Grant Type
Non-Reimbursable
Bill Number
SB01-157, HB02-1152, SB03-110, HB04-1221, SB05-084, HB06-1313, SB07-122
Prepared By
CDM
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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 />
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