INFLUENCE OF HEADWATER STREAM MORPHOLOGY ON STREAM TEMPERATURE REGIMES

Urbanization and associated impervious surface cover can warm stream temperatures by enhancing overland flow from impervious, exposed surfaces and decreasing sheltering from vegetation. Urban runoff can also cause channel incision, potentially altering hyporheic and groundwater flow paths, which may increase contributions of deeper groundwater to stream baseflow. Restoration projects that modify channel morphology but not impervious cover may store warm runoff while decreasing contributions from deep groundwater. Stream remediation decisions, therefore, should consider the impacts of stream morphology on water temperatures. To evaluate this question, we monitored three types of headwater streams: incised, non-incised, and constructed wetland outlet streams. Water level and stream temperature were monitored at five-minute intervals over several years at upstream and downstream sites in both the non-incised and incised first-order streams, the inlet, interior, and outlet wells of the wetland, and the outlet stream. Closely spaced longitudinal stream temperature surveys were conducted in autumn and spring periods along both headwater streams. Although the seasonal range of stream temperatures was similar for both streams, the incised stream exhibited cooler temperatures year-round, especially during summer. The wetland outlet stream exhibited higher warm-season and lower cool-season temperatures than the wetland inlet streams, suggesting that the wetland enhanced seasonal water temperature variability rather than mixing surface and groundwater sources. Storm runoff reflected air temperature during accompanying storm events, restricting stream temperature responses to within the range of diurnal variation. Surface water and groundwater temperatures were more coupled during cool seasons. The longitudinal stream temperature survey indicated low variability along the incised stream during the warm season, reflecting groundwater contributions from below rooting depths, but variable along-stream temperatures during the cool season, when water tables were higher.