Presentation Title

Dam, it's hot in here! Human- vs. beaver-constructed dams and stream temperature

Abstract

Stream temperatures in Johnson Creek and its tributaries often violate the standard established by Oregon DEQ for rearing salmonids (18°C). Staff from the Johnson Creek Watershed Council (JCWC) and the City of Gresham sought information on what contribution to this might be coming from inline ponds, and whether pond type (human- vs. beaver-made) would affect this. JCWC placed HOBO temperature loggers up- and downstream of eight private, human-made ponds, located throughout the watershed, in summer 2018, and took episodic field measurements of temperature in the ponds themselves; City of Gresham staff did the same for both human- and beaver-constructed ponds over several years. We found all human-made ponds for which we could make a determination increased stream temperature; the difference was as much as 9°C. Beaver ponds, by contrast, either had no effect, or , in some cases, resulted in cooler temperatures downstream. Both pond types showed thermal stratification, based on field measurements, with even shallow depths (0.5m) as much as 1°C cooler than surface temperature. In several cases, stream temperatures above human-made ponds were consistently below the DEQ temperature standard, while downstream temperatures exceeded the standard for much of the study period. Human-constructed inline ponds appear to contribute significantly to temperature Total Maximum Daily Load (TMDL) violations in the Johnson Creek watershed; inline beaver ponds may have a net positive effect on temperature.

Subjects

Animal ecology, Hydrology, Water quality

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Dam, it's hot in here! Human- vs. beaver-constructed dams and stream temperature

Stream temperatures in Johnson Creek and its tributaries often violate the standard established by Oregon DEQ for rearing salmonids (18°C). Staff from the Johnson Creek Watershed Council (JCWC) and the City of Gresham sought information on what contribution to this might be coming from inline ponds, and whether pond type (human- vs. beaver-made) would affect this. JCWC placed HOBO temperature loggers up- and downstream of eight private, human-made ponds, located throughout the watershed, in summer 2018, and took episodic field measurements of temperature in the ponds themselves; City of Gresham staff did the same for both human- and beaver-constructed ponds over several years. We found all human-made ponds for which we could make a determination increased stream temperature; the difference was as much as 9°C. Beaver ponds, by contrast, either had no effect, or , in some cases, resulted in cooler temperatures downstream. Both pond types showed thermal stratification, based on field measurements, with even shallow depths (0.5m) as much as 1°C cooler than surface temperature. In several cases, stream temperatures above human-made ponds were consistently below the DEQ temperature standard, while downstream temperatures exceeded the standard for much of the study period. Human-constructed inline ponds appear to contribute significantly to temperature Total Maximum Daily Load (TMDL) violations in the Johnson Creek watershed; inline beaver ponds may have a net positive effect on temperature.