Trends and Controls on Summer Surface-Water Temperatures in Salmonid-Bearing Headwater Streams in Two Common Geomorphic Settings, Kenai Peninsula, Alaska

Stream temperature is an important physical characteristic of headwater streams that plays a critical role in the presence and health of juvenile salmonids. Headwater stream temperature was documented in two geomorphic settings on the Kenai Peninsula, Alaska, focusing on the variation in temperature induced by diffuse groundwater discharge and variable air temperature. Eighteen headwater stream reaches were studied in four watersheds, with 11 drainageway sites and seven discharge-slope sites. In drainageway sites, low-gradient streams flow through broad valleys with groundwater-fed fen wetlands; in discharge-slope sites, high-gradient streams flow through narrow valleys with groundwater-fed slope wetlands. At all 18 sites, hourly stream temperatures were measured for one year. At one drainageway and one discharge-slope site, groundwater temperatures, stream stages, and groundwater heads in the local groundwater flow systems were also measured hourly for a year. A process-based, mechanistic surface-water temperature model (SSTEMP) was used to identify the importance of surface-water temperature controls, particularly the role of groundwater discharge and air temperature, at the two highly instrumented sites. The contribution of groundwater to flow was calculated using a mass-balance water quality mixing model. Groundwater discharge represented 40-60 percent of total stream flow in both geomorphic settings. However, stream and groundwater temperature characteristics differed between the two geomorphic settings. Stream and groundwater temperatures were tightly coupled in the discharge-slope site but not in the drainageway site. Furthermore, warmer stream temperatures occurred at the drainageway site compared to the discharge-slope site, especially during the late summer. SSTEMP simulations indicate that diffuse groundwater discharge does not greatly affect changes in stream temperature at either geomorphic setting, corroborating field observations that focused discharge (e.g., springs and tributaries) help moderate stream temperatures. SSTEMP simulations suggest that air temperature combines with inflowing stream temperature to control stream temperature variations in both geomorphic settings. SSTEMP simulations also indicate that stream temperatures are warmer and increase faster in drainageway sites due to lower-velocity flow and less shading. SSTEMP offers a unique opportunity to examine the control mechanisms of stream temperature in different environmental settings and begin to develop a deeper understanding of the driving mechanisms on a local or reach scale.