Effects of riparian revegetation on stream temperature in the Walla Walla basin

Habitat degradation across the Pacific Northwest has resulted in the decline of stocks of salmonids such as bull trout, steelhead trout, and Chinook salmon. High stream temperatures play a significant part of this degradation by causing behavioral and physiological changes or causing direct mortality to salmonids. In the Walla Walla basin of southeastern Washington there are numerous streams which have been designated as polluted by high temperatures under the Clean Water Act during the summer months of June through September. In response local nonprofit organizations have implemented riparian restoration projects which seek to lower heat inputs into streams by removing non-native vegetation along stream banks and replacing it with native foliage which provides shade and other cooling functions. I used data from in-stream dataloggers to analyze temperature trends in the years following the installation of revegetation projects. Data from loggers near the sources and mouths of Garrison Creek and Caldwell Creek were compared to determine trends in downstream temperature change over time. In Garrison Creek there was a trend towards increased downstream heating in August and September between 2013 and 2017. In Caldwell Creek there has been a slight trend towards increased downstream cooling in June and August between 2015 and 2017. Despite the riparian restoration projects, temperatures in both creeks still regularly rise high enough to maintain their classification as polluted under the Clean Water Act (CWA).