Chapter 5.9, Page 2 of 3

Alluvial Aquifer

Alluvium within the Dolores and San Miguel River basins is comprised of typical Quaternary alluvial valley fill. These deposits consist of gravel, sand, silts, clay, and various mixtures. The alluvial extent is limited to areas near the rivers and their tributaries and disappears entirely in areas where active canyon downcutting occurrs. Dolores River and San Miguel River alluvium are not hydraulically connected, except where the two rivers meet. The greatest extent of alluvium along the southern portion of the Dolores River is located upstream from the town of Dolores and McPhee Reservoir.

The depths of the alluvial wells of record range from 9 to greater than 140 feet below ground surface. Ninety percent of the alluvial wells have been completed at depths less than 120 feet, with the majority of wells completed at depths less than 70 feet. The mean completion depth is 66 feet below ground surface.

Water Levels/Aquifer Characteristics

Little data are available for the Dolores and San Miguel River basin alluvial aquifers. A search of The Colorado Division of Water Resources well permit database yielded just under 100 alluvial wells of record completed to depths of less than 140 feet. Reported water levels for various well depths range from 2 to 90 feet in the two river basins with springs present in some locations. Ground water in the alluvium is under water table, or unconfined, conditions in both river basins.

The alluvial aquifers are only capable of yielding low to moderate quantities of ground water. For the wells of record within the DWR database, reported yields range from 1 to 200 gallons per minute (gpm). Over 90 percent of the wells completed in the associated river alluvium yield less than 50 gpm with the average well yielding only 22 gpm.

Water Use/Withdrawals

Public water supply is the primary use of ground water in San Miguel and Dolores Counties, whereas agriculture is the primary use of ground water in Montrose and Mesa Counties. Alluvial ground water is most commonly used for domestic and stock watering purposes, although irrigation could be accomplished in the near-river areas.

Chapter 5.9, Page 2 of 3