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.  We quantified the relative effects of two of these factors, riparian canopy cover and groundwater inflow, on stream temperatures at the reach scale.  We 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.  We 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, we manipulated the model to examine the relative impacts of varied levels of riparian canopy cover (0-100%) and groundwater inflow (0-50%) on predicted stream temperatures.
Results/Conclusions

SSTEMP predicted reach temperatures well across the range of conditions encountered, although it generally predicted daily mean stream temperatures more accurately than daily maximum temperatures.  Model manipulations of groundwater inflow and canopy cover levels showed consistent trends in affecting stream temperatures.  Under peak summer air temperatures, the maximum change in each factor altered daily mean stream temperatures by ~ 2.5°C.  By contrast, maximum canopy cover decreased maximum stream temperatures by ~ 10°C, while maximum groundwater inflow decreased maximum stream temperatures by ~ 2°C.  These results suggest that while canopy cover is the more dominant factor in determining local maximum stream temperatures, groundwater inflow is also an important factor in summer stream temperature moderation, particularly when streams are approaching thermal thresholds and already have full canopy cover.