Restoring riparian forests as a tool to mitigate effects of future climate change on water
temperatures in salmon-spawning streams of the interior Columbia River Basin
Key Point: Restoring riparian forests on streams where historic land uses have created open meadows
could reduce maximum stream temperatures by as much as 7 oC relative to current conditions, even
under a future climate when air temperatures are 4 oC warmer than today.
Summer maximum stream temperatures are near thresholds of thermal tolerance for salmon and trout in
many streams throughout the interior Columbia River Basin. Salmon and trout populations in many of
these streams are severely depressed, resulting in efforts to restore stream and riparian habitat. Climate
change raises serious questions about the long-term outcomes of restoration because projected warming
could make many of these streams and rivers uninhabitable for salmon and trout within a few decades.
We used the mechanistic stream temperature model, HeatSource, to examine future changes in stream
temperature on the upper Middle Fork John Day River resulting from:
1) a +4 oC increase in air temperature;
2) ±30% changes in stream discharge from both changes in irrigation withdrawals and climatechange related loss of winter snowpacks;
3) four riparian vegetation scenarios:
a) current conditions where effective stream shade averages 19%;
b) a post-wild fire scenario with maximum vegetation height of 1 m and 10% canopy density
resulting in 7% effective stream shade;
c) an intermediate condition representing a young-open forest or tall-shrub dominated vegetation
with trees or shrubs 10-m tall and with 30% canopy density resulting in 34% effective shade;
d) a restored riparian forest with trees 30-m high and canopy density of 50% resulting in 79%
effective stream shade.
Our model results showed the composition and structure of riparian vegetation were the single biggest
factor determining future stream temperatures. In contrast, changing air temperature or stream discharge
had relatively small influence on future stream temperatures. The post-wildfire and the currentvegetation scenarios were warmer than today, but in both cases, effective shade was low, so the stream
was sensitive to air temperature increases due to climate change. The intermediate restoration,
simulating a young-open forest or a tall-shrub dominated riparian zone, was slightly cooler than today.
The biggest change resulted from restoring the riparian forest which decreased summer maximum
temperatures by ~ 7 oC.
Current vs Future Stream Temperatures
Figure 1: Results of model simulations using
HeatSource calibrated to a base-year of
2002. Current conditions are shown as a bold
black line. Projections for 2040 are shown as
colored boxes, and represent ensembles of
model runs with a single riparian vegetation
scenario. Each box includes projections for
high, current, and low discharge. Thus, the
height of each box reflects the influence of
±30% changes in discharge.
Contact: Steve Wondzell, PNW Research Station, swondzell@fs.fed.us; 541-758-8753