STREAM/AQUIFER INTERACTIONS IN GREAT SAND DUNES NATIONAL MONUMENT, COLORADO: INFLUENCE ON INTERDUNAL WETLAND DISAPPEARANCE

The Great Sand Dunes National Monument (GRSA) is located within the San Luis Valley in south-central Colorado. Previous analysis of aerial photographs showed that between 1937 and 1995 more than 100 interdunal wetlands disappeared. For this study we used historical analyses of regional stream flows, water level measurements from groundwater monitoring wells, stream stage and flow measurements, analyses of ²H and ¹⁸O in stream and groundwater, and groundwater modeling to determine the cause(s) of the disappearance of the wetlands.

The study area within the GRSA is bordered on the north and west by Sand Creek, an intermittent stream fed by snow melt from the Sangre de Cristo Mountains to the east. Surface and groundwater levels and stable isotope analyses illustrate the interaction between Sand Creek and the unconfined aquifer in the area where wetlands occurred. When Sand Creek is flowing, seepage through its bed raises the water table and creates a groundwater mound beneath the creek. The seasonal development and dispersion of this mound propagates pressure waves through the aquifer that influence groundwater levels up to 2 km from Sand Creek. This eastward movement of groundwater is confirmed by the stable isotope data which plot on an evaporation line that intercepts the local meteoric water line.

Modeling of the groundwater mound suggests that wetland disappearance was due to climatic fluctuations during the 20th century and downcutting of the Sand Creek channel. Below average stream flow between 1950 and 1980 reduced the seasonal duration of Sand Creek flow across the dune complex, minimizing groundwater mound development. During dry years the water levels dropped approximately 1 m leading to the drying of the interdunal wetlands. Incision of the Sand Creek channel reduced the height of the groundwater mound ~2.5 m, reducing seasonal water table fluctuations nearly 0.2 m at wetland sites. Long period wet and dry cycles have much greater effect on the water table elevation across the dune complex than channel incision, leading us to conclude that the interdunal wetlands are ephemeral features that exist only during wet periods.