IMPACT OF BEAVER DAMS ON STREAM TEMPERATURE IN THE KNIFE RIVER WATERSHED, MINNESOTA USA

We collected and analyzed temperature records from eight streams in the Knife River Watershed (KRW) of northern Minnesota to determine whether the removal of beaver dams affects stream temperature. Beaver dams are being removed from streams in the KRW in order to restore stream habitat by, in part, decreasing stream temperatures for steelhead/rainbow trout present there. We analyzed data from 24 stream sensors placed in eight paired streams, two streams each at four sites, in the KRW during the summer months of 2018 and 2019. In July and August 2018, beaver dams were present upstream of the sensors on all eight streams; before the 2019 summer season, beaver dams were removed from one stream at each of the four sites. For each site, we: 1) calculated the average daily air and stream temperatures for the months of July and August, 2) determined the cumulative number of hours spent in temperature ranges deemed stressful (22.01-25°C) or fatal (>25°C) for steelhead/rainbow trout, and 3) compared the number of hours spent at stressful and lethal temperatures in the streams that had beaver dams (“Beaver” streams) for both years of the study to those that had the dams removed (“Treatment” streams) in the second year. Overall, 2019 was cooler than 2018 for both air and water; the average July-August air temperatures across all locations were 18.6°C and 18.5°C in 2018 and 2019 respectively, while the average daily July-August water temperatures were 18.0°C and 17.4°C in 2018 and 2019 respectively. Most streams experienced more cumulative hours in water temperature zones that were stressful or lethal to trout in 2018 (9% on average for all 8 streams) than in 2019 (7% on average for all 8 streams). Treatment streams were also generally warmer than their adjacent Beaver streams in both years of the study. However, in 2018, Treatment streams spent, on average, 1.7 times as many hours at stressful and lethal temperatures than Beaver streams, while in 2019, Treatment streams spent 11.3 times as many hours in these water temperature zones than Beaver streams. These preliminary results from this short-term study indicate that removing beaver dams (and their resulting beaver ponds) in the KRW does not guarantee cooler downstream water temperatures and may actually result in an increase in stream temperature compared to when dams are present.