By Darrel Dunn, Ph.D., PG, Hydrogeologist 

(View Résumé 🔳)

San Luis Valley Geology

This web-page is a brief non-technical, general description of the San Luis Valley, Colorado, groundwater system and its management.  The valley is a large area in southern Colorado flanked by the Sangre de Cristo Mountains on the east side and the San Juan Mountains on the west side.  It is about 100 miles long in the north-south direction and about 50 miles wide.  It is one of the basins in the Rio Grande Rift.  The valley floor is flat except for the San Luis Hills and a few other small hilly areas south of Alamosa.  The valley floor is underlain by deposits called Valley Fill (aka basin-fill or Santa Fe Group) that are as much as 21,000 feet deep.  The Valley Fill is composed of alluvial gravel, sand, silt and clay.  It also contains buried beds of volcanic flows and other volcanic deposits that extend into the western part of the valley from the San Juan Mountains.  The Valley Fill is underlain by low permeability rock (granite, gneiss, and schist).  The eastern border with the Sangre de Cristo Mountains is a major break in the Earth's crust (fault zone) where the rocks on the valley side of the break have moved downward thousands of feet relative the the rocks in the Sangre de Cristo Mountains.  Alluvial fans have been deposited around the edge of the valley by streams flowing from the mountains.  The most extensive is the Rio Grande fan that covers a large area north of Monte Vista and Alamosa.

San Luis Valley Surface Water

The surface water in the San Luis Valley comprises the Rio Grande River and its tributaries, small streams flowing from the surrounding mountains, lakes, reservoirs, and canals.  The Rio Grande River enters the valley from the San Juan Mountains and flows across the southern part of the valley into New Mexico.  It drains the southern part of the valley.  The northern part of the valley is a closed basin wherein streams carry water toward the San Luis Lakes and playas in the lowest part of the valley (called the "sump").  Water is lost from the lakes by evaporation in the arid climate.

San Luis Valley Groundwater System

The groundwater system of the San Luis Valley is affected by agricultural irrigation.  Irrigation is by diversions from the Rio Grande River and other streams, and by pumping groundwater with large-capacity wells.  Many of the large-capacity wells serve central pivot irrigation systems.  Some have discharged water into canals.  A large part of the Rio Grande's water is diverted into the canal system.  The application of the irrigation water to crops and leakage from irrigation canals maintains a high water table elevation throughout most of the valley.  In some areas waterlogging has been prevented by drainage ditches, and drainage ditches have been used to control the depth to the water table for agricultural purposes.  Irrigation canals can convert to drainage when they are unused.  The water table is within the upper part of the Valley Fill that is called the unconfined aquifer.  In the northern and central parts of the valley the unconfined aquifer is partially separated from the lower part of the Valley Fill by 10 to 80 feet of discontinuous clay layers and the lava flows at depths of 50 feet to over 100 feet.  These low permeability lawyers restrict vertical groundwater movement.  The Valley Fill below the clay layers is called the confined aquifer.  The confined aquifer is connected to areas of groundwater recharge at relatively high elevations around the perimeter of the valley where the clay layers are not present.  This connection leads to water pressure in the confined aquifer in the central part of the valley sufficient to cause wells to flow at the ground surface.  The flow from these wells is controlled to prevent waste of water.  However, if any of the older flowing wells have faulty casing they can leak to the unconfined aquifer.  Wells in the unconfined aquifer yield as much as 3,000 gallons per minute, and wells in the confined aquifer yield as much as 4,000 gallons per minute.  In addition to irrigation wells, there are geothermal wells in the San Luis Valley with water temperatures up to 120 degrees Fahrenheit.  The groundwater system in the San Luis Valley is complex and largely controlled by irrigation practices.

San Luis Valley Sustainable Groundwater Management

The groundwater in the San Luis Valley is managed with the objects of sustaining the groundwater resource and satisfying the requirements of the Rio Grande Compact.  A major challenge is delivering water to New Mexico and Texas via the Rio Grande River in accordance with the compact.  Efforts to maintain Rio Grande River flow and contribute to sustainable groundwater use have included:

The future of the San Luis Valley agriculture seems to be dependent on optimal management of  irrigation and the groundwater system.  Astute use of up-to-date , fully functional and integrated surface water and groundwater modeling software can contribute to optimal management of  water resources.

Posted 4/1/2021.  Revised 6/3/2023.