Chapter 5.8, Page 2 of 3

Alluvial Aquifer

Each of the rivers in the San Juan River system contains alluvial valley fill and extensive terrace deposits. As with most alluvial deposits, they typically consist of varying amounts of gravel, sand, silt, and clay, depending upon distance from the mountain front and location within the depositional system. In general, the San Juan River system alluvium does not exceed 150 feet in thickness, with most areas containing less than 100 feet.

This photo displays the width of the alluvial valley in the meandering
reach of the Animas River north of Durango.
Photo by C. Carroll, CGS.

Of the alluvial wells of record, over 90 percent are completed at depths less than 170 feet, with an average depth of 103 feet. Reported alluvial well depths ranged from a minimum of 2 feet to greater than 200 feet.

Water Levels/Aquifer Characteristics

Water levels within the alluvial aquifer vary tremendously depending upon the stream reach or tributary, saturated thickness of the aquifer, vertical position along the stream reach, and horizontal position within the alluvial valley. Reported depths to water range from a minimum of 1 foot to a maximum of 181 feet below ground surface.

Reported well yields are typically low, ranging from 0 to 50+ gallons per minute (gpm) with a mean yield of 17 gpm. Eighty-five percent of the alluvial wells of record have well yields of less than 18 gpm, suggesting that these wells are used predominantly for domestic purposes and livestock watering.

Little data on the hydraulic properties of the San Juan River system alluvium are available. Reported hydraulic conductivities from the river system upstream of Farmington, New Mexico range from 0.006 to 200 feet per day.

Water Use/Withdrawals

Alluvial ground water from the San Juan River system has typically been utilized for agricultural irrigation, stock-watering, and domestic purposes. Irrigated lands are common along the river valleys and irrigation return water accounts for much of the alluvial recharge along the La Plata, Animas, Florida, and Los Pinos Rivers. Valley fill deposits provide an important source of potable water for many rural residences, although total withdrawals from the alluvial aquifer are small when compared to the bedrock aquifers and surface water.

Chapter 5.8, Page 2 of 3