What Matters Most: Are Future Stream Temperatures More Sensitive to Changing Air Temperatures, Discharge, or Riparian Vegetation?

Abstract

Simulations of stream temperatures showed a wide range of future thermal regimes under a warming climate — from 2.9°C warmer to 7.6°C cooler than current conditions — depending primarily on shade from riparian vegetation. We used the stream temperature model, Heat Source, to analyze a 37‐km study segment of the upper Middle Fork John Day River, located in northeast Oregon, USA. We developed alternative future scenarios based on downscaled projections from climate change models and the composition and structure of native riparian forests. We examined 36 scenarios combining future changes in air temperature (ΔTair = 0°C, +2°C, and +4°C), stream discharge (ΔQ = −30%, 0%, and +30%), and riparian vegetation (post‐wildfire with 7% shade, current vegetation with 19% shade, a young‐open forest with 34% shade, and a mature riparian forest with 79% effective shade). Shade from riparian vegetation had the largest influence on stream temperatures, changing the seven‐day average daily maximum temperature (7DADM) from +1°C to −7°C. In comparison, the 7DADM increased by 1.4°C with a 4°C increase in air temperature and by 0.7°C with a 30% change in discharge. Many streams throughout the interior western United States have been altered in ways that have substantially reduced shade. The effect of restoring shade could result in future stream temperatures that are colder than today, even under a warmer climate with substantially lower late‐summer streamflow.

Riparian canopy openings on mountain streams: Landscape controls upon temperature increases within openings and cooling downstream

“…Reference temperatures in all these streams were near or above temperature stress thresholds for trout. Therefore, summer stream temperature management is a critical habitat issue, particularly in the face of continued warming of the climate (e.g., Wondzell et al, ). Understanding of the effects of riparian canopy openings on downstream temperatures and stream ecosystem processes is critical for developing policies for maintaining coldwater habitat…”

“…Though forested riparian buffers have multiple ecological functions such as reducing sediment loads from upslope disturbances and contributing large woody debris to channels (Sweeney & Newbold, ), they have been shown to substantially mitigate stream temperature effects of watershed forest harvest or land use conversion (e.g., Boothroyd, Quinn, Langer, Costley, & Steward, ; Broadmeadow, Jones, Langford, Shaw, & Nisbet, ; Gomi, Moore, & Dhakal, ; Groom, Dent, Madsen, & Fleuret, ). Riparian restoration may be a key to mitigating climate change effects on streams (e.g., Wondzell et al, ). However, as stream channels become wide relative to canopy overhang and tree height, riparian forest becomes less and less important to stream energy budgets, so temperatures in smaller streams are inherently more sensitive to riparian cover (e.g., DeWalle, ; Gaffield, Potter, & Wang, ; Li, Jackson, & Kraseski, ).…”

Using a Mechanistic Model to Develop Management Strategies to Cool Apache Trout Streams under the Threat of Climate Change

“…; Wondzell et al. ). Existing stream temperature models, which are used to evaluate specific management scenarios to cool streams, usually in response to anthropogenic activities (e.g., urbanization, livestock, or logging), could help to devise management actions for cooling specific streams that will warm due to climate change.…”

“…; Wondzell et al. ) that increasing shade provided by riparian vegetation is one of the most effective, feasible means to keep streams from heating. Planted vegetation has a minimal effect on stream levels when applied sparingly as buffer strips and not watershed treatments.…”

“…), and a variety of heat inputs (Wondzell et al. ) affect streams and could be used to plan for climate change. Use and further development of mechanistic models might allow site‐specific recommendations to be developed by those tasked with managing stream temperature changes on individual stream reaches.…”