San Francisquito Creek Habitat Monitoring Project, Final Report Stream Temperature Characterization

Stream water temperature regimes are determined by the complex interplay of various environmental factors, including prevailing meteorology, surface and subsurface flow patterns, and local riparian canopy structure as it affects solar exposure. We examined temperature regimes with respect to conservation of aquatic organisms for San Francisquito Creek (SFC) watershed (San Francisco Peninsula, CA) using a stream temperature characterization protocol that predicts water temperature regimes for any time period and location The protocol predicts stream temperature dynamics based on energy balance, with a focus on shortwave input from incoming solar radiation (insolation). Analyses synthesize measurements of meteorology from nearby weather stations, stream flow and water temperature from gaging stations, water temperature from a distributed sensor network, riparian canopy structure and solar exposure from hemispherical photography, and stream morphology from field characterization and geographic information system (GIS) analysis. For SFC, water temperature generally tracked air temperature, with significant lags (~ 4+ hr) and local effects. Subsurface flow through gravel beds decreased temperature (2-3° C decrease) and greatly lowered temperature variability. Stream reaches with open riparian canopy had high insolation and displayed relatively high temperature variability (up to 5° C differential from baseline), whereas reaches with closed canopy had low insolation and displayed modest temperature variability (0.5-1.0° C differential). Simulated tree removal demonstrates the power of the tool to evaluate human and natural impacts of riparian canopy modification. Analysis of the July 2006 heat wave demonstrates the value of the protocol for assessing impacts of extreme weather events. Management of SFC stream habitat to include a diversity of suitable temperature regimes is essential for conservation of species such as steelhead trout (Oncorhynchus mykiss), which requires relatively cool conditions, and California red-legged frog (Rana aurora draytonii) and western pond turtle (Clemmys marmorata), which require warmer conditions. Our stream temperature characterization protocol not only is valuable for the SFC watershed, but also can be applied to a broad spectrum of streams for habitat assessment, for conservation and restoration, and for examination of potential impacts of climate change on stream ecosystems.