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<br />I <br />I <br />I <br />I <br />I <br />II <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />il <br />I <br /> <br />SECTIONTHREE Anected Environments and Environmental Consequences <br /> <br />The build alternatives have low potential for adverse impacts on the human or natural <br />environment because they would primarily affect areas that were previously disturbed by <br />development activities and/or previous flood events. This EA focuses on those resources where <br />impacts could result. <br /> <br />3.1 GEOLOGY AND SOILS <br /> <br />Logan County is located in the high plains of northeastern Colorado near the center of the Great <br />Plains physiographic province. The greater part of the county is a dissected plain which has been <br />mantled with thin silty loess deposits, thick eolian sandy deposits, and locally reworked old <br />alluvial materials. The valley of the South Platte River varies in width from I to 4 miles. Nearly <br />level bottomlands and low terraces with fluctuating water tables are located adjacent to the river. <br />The well-drained higher terraces, mainly on the north side of the river, are some of the best- <br />irrigated soils in the area. Exhibit 9 shows the topography of the area. <br /> <br />The Soil Conservation Service (SCS) who now is the Natural Resources Conservation Service <br />(NRCS) conducted a detailed survey of soils in Logan County between 1965 an 1974 (SCS <br />1977). Ten soil mapping units (Aida loam, Fluvaquentic Haplaquolls, Fluvaquents, Glenberg <br />fine sandy loam, Haverson loam, Hayford silty clay loam, Lebsack silty clay loam, Nunn loam, <br />Satanta loam, and Westplain silty clay loam) are present ",~thin the project area (SCS 1977). A <br />brief description of each soil mapping unit is provided in Appendix B. <br /> <br />With the exception of the Fluvaquentic Haplaquolls and Fluvaquent soil mapping units, all soil <br />mapping units within the project area are used extensively to produce irrigated crops (SCS 1977). <br />The Fluvaquentic Haplaquolls and Fluvaquent soil mapping units and small areas ofthe other <br />mapping units are used for livestock grazing. Soils within the project area have one or more <br />characteristics which reduce their favorableness for the construction of earthen embankments, <br />dikes, and levees (SCS 1977). These undesirable characteristics include seepage, piping, poor <br />compaction, shrink-swell, and low strength. Each of these factors would be evaluated in detail <br />during the final engineering design of the project. Special design and/or construction procedures <br />would be implemented as needed. <br /> <br />3.1.1 Alternative 1 . No Action <br /> <br />The No Action Alternative would not affect geology and soils located in the project area since <br />this alternative does not include any restoration or improvement activities. However, soil erosion <br />could continue to occur during future flooding events. <br /> <br />3.1.2 Alternative 2 . Improve Routing of Pawnee Creek Flood Flows (Proposed <br />Action) <br /> <br />The levee and channel construction activities associated with this alternative would not impact <br />the geology of the project area. Overall, approximately 45 acres of soil would be disturbed <br />during vegetation removal, grading, and levee and channel construction. Of the disturbed soils, <br /> <br />3-1 <br />