Chapter 5.7, Page 2 of 3

Alluvial Aquifer

Mapped alluvial sediments in the basin are constrained to the immediate vicinity of the drainage valley. In Colorado, the South Fork Republican and the Arikaree Rivers are within the Ogallala Formation, although the Arikaree River has downcut into the Pierre Shale west of US Highway 385. Frenchman Creek also passes through the Arikaree Formation. The hydrogeologic units in the Republican/Arikaree River basin are Pleistocene alluvial and eolian deposits. The alluvial deposits consist of poorly sorted gravel, sand, and clay with caliche. Eolian sand and silt cover much of the land surface outside of the stream valleys and overlap the alluvial deposits.

The alluvial deposits in these river basins are generally less than 100 feet in thickness, and are often in hydraulic connection with the underlying bedrock formation. Reported well depths generally range from 20 feet to more than 65 feet. Ninety percent of the alluvial wells of record are completed at depths of less than 60 feet, with a mean depth of 46 feet.

Water Levels/Aquifer Characteristics

Reported depths to water, below ground surface, for the alluvial wells of record range from 5 to 64 feet, with an average of 18 feet. For the 57 wells of record with reported water levels, the water table generally lies between 10 and 20 feet below ground surface.

The reported alluvial well yields in the Republican/Arikaree River basin display a distinct bimodal distribution with slightly over 60 percent of the wells producing less than 55 gallons per minute (gpm). These wells are suitable for domestic use, livestock watering, and small acreage irrigation. Reported well yields from 250 to 950 gpm are also common. The higher yield wells are most likely utilized for large-scale irrigation.

Hydraulic conductivity values range from 30 to 270 feet per day, and annual recharge to the aquifer is estimated to range from 0.25 to 0.50 inches.

Chapter 5.7, Page 2 of 3