STREAM INCISION AND SAGEBRUSH EXPANSION IN THE SIERRA NEVADA MOUNTAINS, CALIFORNIA: A HYDROLOGIC PERSPECTIVE

Within the last hundred years, landscapes across the western United States have experienced stream incision and vegetation change. In wet meadows in the southern Sierra Nevada Mountains, the streams are incising and sagebrush (Artemisia rothrockii) has displaced meadow vegetation, dramatically changing a unique alpine habitat. This study examines the relationship between vegetation and water table depth and the impacts of stream incision on the meadow hydrology in order to determine whether the hydrologic changes associated with stream incision are responsible for sagebrush encroachment. To document these relationships, Bullfrog Meadow on the Kern Plateau, Sierra Nevada Mountains was surveyed and 34 groundwater-monitoring wells were installed. Water table measurements and hydraulic conductivity data were collected during the summer of 2003. Data on stream discharge, sediment type, and vegetation distribution were also gathered. As observed in other studies, there is a strong correlation between vegetation type and water table depth. The water table is on average 50 cm below meadow vegetation within the study area, 120 cm below sage meadow vegetation, and on average 200 cm below sagebrush vegetation. In the incised region of the meadow, stream incision lowered the water table adjacent to the stream by up to 150 cm and approximately 50 cm further from the incised stream. The groundwater flow direction, water table gradient and stream discharge have also been altered by the incision. These results indicate that the hydrologic effects of stream incision on the groundwater system may significantly impact groundwater storage, affecting late summer soil moisture and vegetation change.