5.8 - SAN JUAN RIVER SYSTEM
The San Juan River originates along the Continental Divide in southern Colorado, just north of the town of Pagosa Springs. The watershed encompasses about 26,000 square miles of Colorado, New Mexico, and Arizona before merging with Lake Powell and the Colorado River system in Utah. The entire San Juan River system in Colorado is located within Water Management Division 7, with the division office in Durango. As of February 2001, over 1,800 wells were on record as being completed in the alluvial aquifers of the San Juan River basin. The majority of the alluvial wells are concentrated in the Los Pinos, Animas, and La Plata River valleys. The occurrence and distribution of alluvium along the San Juan River system are shown in yellow on Figure 5.8-1.
In Colorado, the San Juan River system is fed by a series of sub-parallel rivers that drain the San Juan and La Plata Mountains. These rivers, from east to west, include the Piedra, Los Pinos, Florida, Animas, La Plata, and Mancos Rivers. Altitudes in the San Juan River system range from greater than 13,000 feet near the headwaters of the San Juan and Piedra Rivers to nearly 4,500 feet where the Mancos River exits the state just east of Four Corners. The San Juan River contributes about 10 percent of the total flow in the Colorado River Basin. The average annual flow exiting the state through the San Juan River represents approximately 7 percent of the state’s total annual stream outflows.
Alluvial aquifers in the San Juan River system are typically limited areally in the higher mountain areas and become more expansive as they exit the mountain fronts. In general, the San Juan and Piedra Rivers form one combined alluvial system, as do the Florida and Animas Rivers. The Los Pinos, Florida, Animas, and La Plata alluvial systems are all used for irrigation and stock watering purposes.