Chapter 5.4, Page 2 of 3

Alluvial Aquifer

The alluvium in the Yampa River basin typically consists of unconsolidated deposits of clay, silt, sand, and gravel. The saturated thickness of the Yampa River alluvium ranges from 10 to 100 feet. In the tributary valleys, the saturated portion of the alluvium is generally less than 20 feet thick. Alluvium can be thin or absent where the streams cross hard, resistant bedrock such as sandstone, and thick and wide where the streams cross less resistant bedrock such as shale.

Sand and gravel deposited as bars during high flows
represent modern alluvium.
Photo by C. Carroll, CGS.

Recharge of the alluvial aquifer occurs mainly from bank storage during spring runoff, leakage of irrigation ditches and laterals, and underflow from sedimentary rock aquifers. The Browns Park and Fort Union Formations (Tertiary age) discharge to the alluvium where the alluvium overlies these formations.

Published depths for alluvial wells in this basin range from 5 to 190 feet and average about 30 feet deep. The Division of Water Resources well permit database contains approximately 337 wells that have been completed in the Yampa River alluvium. Ninety percent of these wells have been completed at depths less than 140 feet, with a mean depth of 63 feet.

Water Levels/Aquifer Characteristics

Published water levels in alluvial wells range from 0 (at land surface) to 41 feet below ground surface, averaging about 10 feet. The alluvium is generally a water table aquifer and water levels will fluctuate seasonally with stages in the adjacent surface water courses.

Yields from alluvial wells in this basin have been reported from five to several hundred gallons per minute with the highest yields from the Yampa River alluvium near Steamboat Springs, Hayden, and Craig. A close inspection of alluvial wells in the Yampa River basin indicates that domestic water supply wells with yields of 15 gpm or less predominate.

Although information about hydrologic aquifer characteristics for the alluvium in the Yampa River basin are scarce, published values for hydraulic conductivity of the alluvium ranges from 1.9 to 28.8 feet per day. These values are typical for fine- to coarse-grained sand.

Chapter 5.4, Page 2 of 3