HYDROSTRATIGRAPHIC AND STRUCTURAL CONTROLS ON STREAMFLOW GENERATION IN SEMIARID, SNOW-DOMINATED, MOUNTAINOUS WATERSHEDS IN THE CHUSKA MOUNTAINS OF THE NAVAJO NATION, NORTHERN NM/AZ

Recent research in the Saguache Creek (CO) and Rio Hondo (NM) watersheds has shown that deep groundwater circulation in the mountain block can contribute significant amounts of groundwater to streamflow. However, the mechanisms controlling deep groundwater circulation are different in these two watersheds. In the Saguache Creek watershed, it is controlled by a combination of primary porosity in nonwelded volcanic tuffs and fracture porosity in welded tuffs (volcanic forces). In comparison, it appears to be primarily controlled by fracture porosity in the Rio Hondo watershed (tectonic forces). Both watersheds conform to a fully 3D conceptual model of groundwater/surface-water interactions incorporating groundwater from deep circulation. Three dimensional flow in mountainous watersheds is fundamentally different than watersheds characterized as two dimensional because 3D watersheds exhibit longer mean residence times in springs and streams. In this research, groundwater/surface-water interactions are investigated in the Whiskey Creek watershed located in the Chuska Mountains of the Navajo Nation along the Arizona-New Mexico border. The Chuska Mountains are primarily comprised of sedimentary bedrock capped by extrusive volcanics. Beginning in 2013, field campaigns were initiated to conduct stream gauging and the collection of water samples for general ion chemistry and stable isotope composition. Samples were collected from precipitation collectors, streams and springs. Our results indicate that most streamflow is generated from groundwater emerging from the top conductive Chuska Sandstone unit. Many springs (n = ~20) emerge from this unit and support streamflow in Whiskey Creek. Gaining/losing conditions are strongly related to hydrostratigraphy as evidenced by a transition from gaining conditions largely found in sandstone layers to losing conditions near the outlet where the Chinle Formation outcrops. This hydrostratigraphic control is similar to behavior found in the El Rito watershed, a mountainous sedimentary watershed located east of the Chuskas in the Tusas Mountains in northern NM. Although 3D interactions do occur in Whiskey Creek and El Rito, hydrostratigraphic and structural controls may limit the occurrence of longer residence times and longer flow paths.