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Abstract Stream thermal regimes are being altered by climate change with consequences for aquatic organisms. Most documented long‐term changes in stream temperature are from large rivers. We know less about water temperature trends for small headwater streams, especially those found in northern landscapes that contain small lakes. We analyzed 36 yr of stream temperature observations from a long‐term watershed study in Ontario, Canada, to understand how headwater streams are responding to climate variability. We found that groundwater‐fed (GWF) and lake‐fed (LF) streams exhibit contrasting responses, as GWF streams warmed in the spring (0.19–0.60°C per decade) and LF streams warmed in the fall (0.39–0.72°C per decade). Both stream types exhibited weak temperature trends in summer and winter. These results highlight that a stream network perspective that includes headwater streams and small lakes, and accounts for seasonal changes in thermal regimes, is important for understanding aquatic ecosystem response to climate variability.

Abstract Small headwater lakes are common water features in northern environments. These small lakes are often reported to have an influence on downstream water quality; however, few studies have addressed the underlying hydrology of these systems and how small lakes influence downstream discharge or how far downstream these influences persist. We show that catchments with small lakes sustain baseflows compared to catchments without lakes. In addition, small lakes have limited influence on the magnitude and timing of peakflow events, except for immediately downstream of the lake where peakflow hydrographs are characterized by low magnitude and long duration. The relative contribution of lake water to downstream discharge can vary widely in time (between 0% and 75%) and be detectable when lakes make up as little as 0.5% of catchment area. This variability and persistence of lake water in stream networks may have important implications for how we interpret water quality patterns downstream of small lakes.