FLOOD09102

• County: Logan
• Community: Sterling
• Basin: South Patte
• Title: Sterling Flood Control Interceptor Channel Project
• Date: 4/1/2002
• Prepared For: Sterling
• Prepared By: URS
• Floodplain - Doc Type: Project

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SEmO.THREE Affected Environments and Environmental Consequences

The EA focuses on those resources where impacts could occur.

3,1 GEOLOGY AND SOILS

Logan County is located in the high plains of northeastern Colorado near the center of the Great
Plains physiographic province. The greater part of the county is a dissected plain which has been
mantled with thin silty loess deposits, thick eolian sandy deposits, and locally reworked old
alluvial materials. The valley of the South Platte River varies in width from I to 4 miles. Nearly
level bottomlands and low terraces with fluctuating water tables are located adjacent to the river.
The well-drained higher terraces, mainly on the north side ofthe river, are some ofthe best-
irrigated soils in the area. Exhibit II shows the topography of the area.

The Soil Conservation Service (SCS), who now is the Natural Resources Conservation Service
(NRCS), conducted a detailed survey of soils in Logan County between 1965 and 1974 (SCS
1977). Eight soil mapping units (AIda loam, Fluvaquents, Haverson loam, Hayford silty clay
loam, Lebsack silty clay loam, Nunn clay loam, Satanta loam, and Nunn loam) are present within
the project area (SCS 1977). A brief description of each soil mapping unit is provided in
Appendix B.

With the exception of the Fluvaquent soil mapping units, all soil mapping units within the project
area are used extensively to produce irrigated crops (SCS 1977). The Fluvaquent soil mapping
units and small areas of the other mapping units are used for livestock grazing. Soils within the
project area have one or more characteristics which reduce their favorableness for the
construction of earthen embankments (SCS 1977). These undesirable characteristics include
seepage, piping, poor compaction, shrink-swell, and low strength. Each of these factors would
be evaluated in detail during the final engineering design of the project. Special design and/or
construction procedures would be implemented as needed.

3,1.1 Alternative 1 . No Action

The No Action Alternative would not affect geology and soils located in the project area since
this alternative does not include any restoration or improvement activities. However, soil
erosion could continue to occur during future flooding events.

3.1.2 Alternative 2. Sterling Flood Control Interceptor Channel (Proposed Action)

The channel construction activities associated with this alternative would not impact the geology
of the project area. Overall, approximately 60 acres of soil would be disturbed during vegetation
removal, grading, and channel construction. Of the disturbed soils, approximately 4 acres have
been previously disturbed by road or railroad development, 55 acres are currently used as
agricultural land, and approximately 1 acre along the South Platte River has riparian vegetation.
Disturbance of approximately 60 acres of soil does not represent a significant impact because
none of the affected soils are unique or have limited abundance in Logan County. In addition,
topsoil within the cropland areas would be stockpiled and spread back over the disturbed areas
after construction has been completed to allow grass to grow. To reduce soil erosion during
construction and to reduce the amount of silt entering the South Platte River, Best Management

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