Too Hot, Too Cold, Just Right: Thermal Modulating Capacity of Different Sources of Groundwater Discharge to Salmon‐Bearing Streams

ABSTRACT Groundwater discharge plays a key role in maintaining habitat suitability for cold‐water fish by generating streamflow and modulating temperatures. However, the thermal influence depends on groundwater source depth and how far groundwater travels from the point of discharge to a receiving stream. In the unglaciated Kenai Peninsula Lowlands (KPL), Alaska, groundwater originates from shallow hillslope groundwater (HGW) or deeper aquifer‐outcrop groundwater (AOGW) sources. We combined 4 years of field observations with a physical model to evaluate the capacity of these groundwater sources to modulate stream temperatures. We additionally conducted geospatial analyses to assess the relative probability groundwater discharge occurs near KPL streams. Hourly data from 18 sites indicate that HGW discharge temperatures vary widely (−1.0°C–11.6°C year‐round) and are similar to stream temperatures, while AOGW discharge temperatures remain relatively stable (3.0°C–4.8°C year‐round) and are warmer than streams in winter and cooler than streams in summer. Using these observations, we modeled the temperatures of HGW and AOGW from their points of discharge along surface‐water flowpaths of varying length prior to mixing with streams. Model results indicate AOGW retains its thermal modulating capacity over longer distances than HGW, which equilibrates more rapidly with atmospheric conditions. Geospatial analyses demonstrate that high groundwater discharge probability occurs near streams in the KPL, with 13% of total groundwater discharge probability occurring within 10 m of streams and ~70% within 200 m. Our results indicate that thermal modulation depends on groundwater source and overland travel distance prior to reaching the stream, with substantial groundwater contributions originating well beyond the immediate vicinity of KPL streams. These findings highlight the importance of groundwater‐surface water connectivity and groundwater source heterogeneity when assessing temperature resilience in salmon‐bearing streams.