Comparing headwater stream thermal sensitivity across two contrasting lithologies in Northern California, USA.

Abstract:

Stream temperature is a critical water quality parameter; however, few studies have assessed longitudinal stream temperature trends at high spatiotemporal resolution in forested headwaters. Thermal regimes in headwater streams are critical to understanding aquatic habitat resilience or resistance to disturbances, including climate change and timber harvesting. We analyzed stream and air temperature data collected during summer 2018 along eight headwater stream reaches located in two regions characterized by distinct lithology, climate, and riparian vegetation. Five streams were located in the Coast Range and three were in the Cascade Range of Northern California, USA. We instrumented each stream with 12 in-stream and 4 co-located air temperature sensors. We compared daily temperature metrics and stream thermal sensitivity to air temperature—the slope of the linear regression line between mean daily stream and air temperature—to quantify atmospheric control on stream temperature. Our results indicated that summer stream temperatures were warmer, but less variable, in Coast Range streams compared to the Cascade Range streams. Streams in the Coast Range were also more sensitive to changing air temperature, despite having greater riparian canopy cover and less diel air temperature variation. The volcanic lithology in the Cascades supported discrete locations of groundwater discharge, which likely dampened thermal sensitivity. Specifically, at three groundwater discharge locations in the Cascade Range, mean stream temperatures decreased by 2.0 °C, 3.6 °C, and 7.0 °C relative to the adjacent upstream sensor. The thin, friable soils in the Coast Range likely contributed baseflow from shallow subsurface sources, which were more influenced by changing air temperatures. Alternatively, the fractured bedrock and coarse soils in the Cascades created discrete locations where groundwater strongly cooled temperatures. Overall, these results reveal greater fine-scale thermal heterogeneity in streams underlain by basalt than sandstone. Future research should consider the effects of riparian timber harvesting on fine-scale temperature dynamics and aquatic habitat viability in headwater streams.