Variable Drought Impacts on Stream Habitat Across the Northwestern United States

The Western United States has been in an extended drought since 2002 which can drive negative impacts on stream habitats supporting native fish and aquatic species. Stream quality, specifically in the context of fish habitat, depends on many variables, including vegetation quality. Drought conditions affect vegetation conditions; however it is unclear how drought conditions impact stream habitat quality for fish. This study examines how stream habitat quality relates to drought across the National Forests of the Northwestern US, as well as exploring whether drought increases have had a significant impact on riparian vegetation over the past 20 years. Using a long-term stream habitat monitoring dataset from the PacFish/InFish Biological Opinion Monitoring Program and satellite-imagery derived Normalized Differential Vegetation Index (NDVI) data, we are able to examine the long term trends in greeness, and the relationship between riparian vegetation condition and stream habitat quality. I found no linear trend of drought impacting riparian vegetation conditions from 2000-2020 in Idaho, Montana, or Oregon National Forests. Linear regression analyses revealed positive relationships between NDVI and stream habitat quality in many, but not all years. Only one National Forest, Beaverhead-Deerlodge in Montana, had an entirely significant relationship with increasing drought causing a decrease in habitat quality. All other National Forests showed some overlap with zero. Riparian vegetation has thus far not been negatively impacted by drought, and stream habitat is largely positively related to riparian vegetation conditions. Further analysis of this data with a greater area of combined NDVI values, as well as the use of more specific NDVI data could provide greater insights into drought patterns across the Northwestern United States, as well as the effect of potential drought on streams or watersheds as a whole.