Final Environmental Assessment for the Tamarack Managed Groundwater Recharge Project

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Proposed ponds and pipelines on PESWA also occur on deep, excessively drained soils. The soil is characterized by blowouts and dune topography. These soils absorb water rapidly but have very rapid internal drainage and low water - holding capacity. Natural fertility is low and there is no surface runoff. Valentine fine sands and Elmere loamy fine sands comprise most of the project area soils.- Proposed well sites border Wet Alluvial land, which is found directly adjacent to the river. This soil type is an accumulation of sand and gravel that contains thin lenses of silty and clayey materials over deep gravel. These materials make up sandbars and islands that are wet most of the time. There are no Prime or Unique Farmlands located on the TRSWA or PESWA. A copy of the NRCS clearance for prime and unique farmlands is included in Appendix D. HYDROLOGY The South Platte River originates along the Continental Divide in Colorado. It flows generally northeast from its headwaters through the Denver metropolitan area continuing i' g northeast through Colorado and into the State of Nebraska near the Town of Julesburg in Sedgwick County, Colorado. As shown in Figure 1, TRSWA is located about 30 miles above the state line and PESWA is located about 4 miles above the state line. The lands comprising TRSWA and PESWA are on both sides of the South Platte River. The South Platte River through both of these State Wildlife Areas is a sand bed alluvial meandering system characterized by numerous braided channels, sloughs, and backwaters. The South Platte River in Colorado and in the vicinity of TRSWA and PESWA is a gaining river system. This means that the alluvial groundwater aquifer drains into the river channel because water levels in the aquifer are higher in elevation than river water levels. The increase in river flows as a result of the return flows from the aquifer can be 5 to 10 cfs per mile as measured from an upstream point to a downstream point. This aquifer exists in highly permeable soils and is supplied by the seepage and deep percolation from existing canals, reservoirs, irrigation, and groundwater recharge programs. Numerous springs and seeps occur along the banks of the South Platte River as the return flows seep back into the river channel. These seeps provide the inflows to the warm water sloughs and backwaters that currently exist at TRSWA and PESWA. Average annual flow amounts of the South Platte River at the Colorado/Nebraska state line are approximately 443,000 acre -feet of water. These flows provide a good surrogate for river flow volumes in the vicinity of TRSWA and PESWA. The annual discharge pattern of the South Platte River at the SWAs is characterized by a spring runoff peak resulting from snowmelt in May and June. On average, this peak amounts to about 40% of the South Platte River's annual volume as measured at the Colorado — Nebraska state line. As mountain snowpack melt and irrigation diversions increase, flows during the summer months of July, August, and September are typically low, though strong thunderstorms can create significant peaks of short duration. On average, 11% of the South Platte River's 7