Modeling relative effects of riparian cover and groundwater inflow on stream temperature in lowland Whatcom County, Washington

Many Pacific Northwest streams have water temperatures that exceed thermal thresholds for salmonids. Supporting and maintaining streams with temperatures below these thermal thresholds requires an understanding of the relationships between the main factors influencing stream temperatures. This study examined the relative effects of two of these factors, riparian canopy cover and groundwater inflow, on stream temperatures at the reach scale. I measured stream temperature, net groundwater exchange, and riparian canopy cover levels in 10 different study reaches designed to comprise a factorial combination of reaches with vegetated and unvegetated riparian buffers, as well as gaining and not-gaining groundwater. I then modeled stream temperatures in each reach with the SSTEMP stream temperature model, and compared model-predicted temperatures to measured stream temperatures during the warmest part of the summer. Finally, I manipulated the model to examine the relative impacts of riparian canopy cover (0-100%) and groundwater inflow (0-50%) on predicted stream temperatures. SSTEMP predicted daily mean reach temperatures well across the range of conditions studied here, although it overpredicted daily maximum temperatures. Model manipulations of groundwater inflow and canopy cover levels showed consistent trends in affecting stream temperatures. Under peak summer conditions and base groundwater (0%) and canopy cover (0%) conditions, predicted mean stream v temperatures warmed by an average of ~ 4°C across all streams. Full canopy cover and 50% groundwater inflow each reduced this predicted warming by ~ 2.5°C when manipulated independently. However, only the combination of both high canopy cover and groundwater inflow actually reduced predicted mean stream temperatures within the study reaches. In contrast, canopy cover had much stronger effects on modeled maximum stream temperatures than did groundwater inflow. Under peak summer conditions, 100% canopy cover reduced predicted downstream warming of daily maxima by ~ 10°C, while 50% groundwater inflow did so by only ~ 2°C compared to base conditions. The results of this study affirm that both canopy cover and groundwater inflow play significant roles in minimizing stream temperatures in summer, and both should be considered when making restoration, land use, and other management decisions