Temperature moderation in a coastal coldwater stream a study of surface water, groundwater and hyporheic zone interaction

A fiber-optic distributed temperature sensor (FODTS) survey was conducted along a 520 m reach of Wednesday Hill Brook (WHB) in Lee, NH, a first order tributary to the Lamprey River. These data were supplemented by stream and streambed temperature and vertical hydraulic gradient data collection at 35 piezometers, and continuous and periodic measurement of tributary, stream, and groundwater temperatures and streamflow. An under-canopy weather station provided on-site meteorologic data, and a LiDAR survey provided high definition land surface topographic data for interpretation of geomorphology. A heat budget model was developed and used to estimate the advective components of heat flow to and from the stream. The FODTS survey describes a stream that experiences a late summer mean temperature drop of over 2°C within the first 150 m and a sustained temperature of less than 13.5°C in the lower 350 m. Multiple local variations in temperature are detected in the lower portion of the reach. Streambed temperatures and hydraulic gradient data suggest that vertical hyporheic exchange is predominantly found in the upper 200 m. Exchange penetrates to 20 cm in most of the reach but is greater than 40 cm in a few locations and plays a large role in temperature reduction. Groundwater discharge in these upper reaches is also substantial and is focused in spring brook discharge areas. A substantial sand and gravel deposit of late glacial origin (Birch, 1989) discharges along the base of the western hillslope near its contact with overlying marine silt and clay (Goldsmith, 1990). A shallow bedrock bowl suggested by an EM survey also underlies this upper catchment area. The bedrock lip coincides with valley constriction and a sudden change in stream direction. Vertical hyporheic exchange decreases downstream. Exchange is most active at instream log and debris dams in the lower reaches. Groundwater discharge along preferential flow pathways is prevalent in this lower catchment area. A heat budget analysis quantifies non-advective influences, net radiation, convection, evaporation, friction and streambed conduction, and advective influences, hyporheic exchange and tributary and groundwater discharge. Temperature gains within 4 sub reaches were dominated by net radiation, which accounted for nearly 50% of heat gain. Convection and evaporation (condensation) made up most of the remaining heat gain. Friction was an insignificant influence. Tributaries added modest heat gains in the lower reaches. Heat losses were dominated by hyporheic exchange (50 to 85%) and groundwater discharge (14 to 40%) in the upper reaches with tributary discharge and streambed conduction making up the balance of heat loss. In the middle and lower reaches, groundwater discharge accounted for 56% of heat loss with streambed conduction making up 37 to 44%. Hyporheic exchange did not provide heat loss in the middle reach and accounted for only 6% of the heat loss in the lower reach. Two localized zones of significant heat loss were identified in the upper 150 m of stream. Here, groundwater and tributary discharge were focused at the outflow of two spring brooks flowing from the western valley. The influences of hyporheic exchange and streambed conduction are maximized where these tributary/groundwater discharge points cool both the stream and the streambed. On a smaller scale, this same symbiotic cooling effect is active in the lower reach where zones of preferential flow discharge cool groundwater to the stream. In this study reach, vertical hyporheic exchange provides the greatest cooling mechanism and groundwater discharge is the underlying temperature control. The temperature delineation made possible by the FODTS stream temperature provided the resolution needed to define focused groundwater discharge and hyporheic cooling zones. This detailed temperature survey tool may re-define our understanding of groundwater discharge regimes. The research also demonstrates the importance of small-scale geomorphic features and hydrologic mechanisms in low order and headwater streams and underscores their value in supporting fresh water ecosystems, nutrient cycling and water resource protection