Groundwater is an important source of baseflow. Baseflow supports minimum flows and living area through dry periods, and moderates surface water temperature. The reductions in baseflow after urbanization can cause degradation of the stream ecosystem. The purpose of this study was to investigate the groundwater-surface water interaction and thermal regime of Clythe Creek, Guelph to illustrate the importance of groundwater/streamflow interaction in determining the health of a stream ecosystem. Piezometers were used to quantify vertical and lateral groundwater flow direction, and surface water temperature measurements were used to characterize and quantify the thermal regime. Groundwater-surface water interaction varied both temporally and spatially between the two geomorphic units. Average summer surface water temperatures were consistently cooler in the downstream portion of the study reach. The importance of groundwater-surface water interactions in supporting stream ecosystems was concluded from this study, and restoration strategies to address threats from urbanization were made. / Ontario Research Fund (ORF)