MONITORING OF GROUNDWATER-SURFACE WATER INTERACTIONS IN SUPPORT OF RESTORATION OF HYPORHEIC PROCESSES IN AN URBAN STREAM, THORNTON CREEK, WASHINGTON

Urban streams typically face at least three forms of degradation: altered hydrology, particularly stormwater runoff; degraded water quality, and; habitat loss. While efforts to restore physical habitat structure are common, restoration of ecological processes, such as restoration of hyporheic interactions, is relatively unexplored. Hyporheic processes are key to maintaining higher water quality because the underlying substrate fosters both biogeochemical reactions and heat transfer with cooler groundwater. This study demonstrates a methodology for characterizing groundwater-surface water interactions on two spatial and temporal scales in order to help design, evaluate performance, and adjust a proposed channel and floodplain restoration project. Reach-scale monitoring consisted of continuous measurements of piezometric head and streambed thermal profile. Results identified gaining, losing and neutral reaches, provided estimates of streambed hydraulic conductivity, groundwater influx and heat transfer rates, characterized diurnal and seasonal patterns of streambed temperature and illustrated the way these patterns vary by location in the stream network. Habitat-unit scale monitoring consisted of extensive measurements of intra-gravel temperatures to identify locations and relative strength of down-welling and upwelling zones. Conceptual design alternatives for one restoration site are examined for consistency with geomorphic setting and to explore implications to the style of hyporheic flow and potential for enhancing hyporheic interactions.