Sustain Our Nation's Forests and Grasslands

Warming western river systems

COLORADO—What will the future hold for western river systems and the cold-water species that depend on them? To answer this question, scientists at the Rocky Mountain Research Station compiled temperature records from nearly 400 monitoring sites along large rivers in the northwestern United States. “Global warming of salmon and trout rivers in the northwestern U.S.: Road to ruin or path through purgatory,” recently published in Transactions of the American Fisheries Society, found that average river temperatures during the summer and early fall months rose about 1 degree Celsius from 1976 to 2015 and that if trends continue, western rivers could warm by an additional 1 degree Celsius by 2050. This warming trend is likely to result in habitat shifts and population changes for many trout and salmon species in these large river systems.

“The data show that global trends are having local impacts on our western rivers,” said Dan Isaak, lead author of the study. “River temperatures are increasing more slowly than air temperatures across the region and cold-water fish should be able to adapt for the foreseeable future; however, some river reaches will gradually become too warm to provide traditional habitats and if the warming trend continues throughout the century, we could see some very different fish communities.”

The study uses historical monitoring data compiled from more than a dozen agencies to describe historical trends in an effort to better predict the future. Scientists also looked at other trends over the same period, which showed warming air temperatures and declining water flows. They translated those trends into potential biological impacts that may occur later this century, focusing on species such as migrating salmon and resident trout like brown trout and rainbow, in nearly 35,000 miles of rivers. This is important because healthy trout and salmon populations are major economic drivers in the region for both recreational and commercial fisheries, and because other, declining fish populations are listed as endangered.

The authors suggest several options that managers could use to help offset warming and preserve cold-water river habitats. These include minimizing water withdrawals from rivers, increasing shade provided by riparian vegetation, enhancing habitat diversity and the number of deep pools that often tap into cold groundwater, releasing cold water from deep storage dams during heat waves, and improving fish passage at dams that block access to cooler river sections. 

Image shows mean river temperatures in the northwestern United States, ranging from less than 12 degrees Celsius to greater than 20 degrees Celsius. Rivers are color-coded for temperature ranges.
Summer temperatures throughout the 35,000 miles of rivers in the northwestern U.S. are diverse but have been increasing steadily in recent decades.
Photo: Rainbow trout in shallow water.
Rainbow trout are one of the popular cold-water sport fish that will have to contend with rising river temperatures this century. Forest Service photo by Brett Roper.