Water Quality Responses to a Semi-Arid Beaver Meadow in Boise, Idaho

Beavers have been instrumental in shaping the North American riverine landscape. However, land use change and beaver trapping have caused large decreases in beaver populations, resulting in fundamental changes to river morphology, hydrology, and biogeochemical function. Effective river restoration and remediation of arid western rivers relies on a comprehensive interpretation of how beaver activity influences water quantity and quality. In this study, I compared two stream reaches with and without beaver dams in a semi-arid watershed, to quantify the effects of beaver activity on hydrology and biogeochemistry. Within each reach, I combined dilution gauging and stream tracer experiments to determine basic hydrologic measures, and analyzed water samples, using ion chromatography, to determine the concentration of major ions. Data was collected from May to July, wherein discharge rapidly declined through both reaches. Magnesium concentrations, [Mg 2+ ], decreased in both reaches, during the eight week period, and suggests [Mg 2+ ] were dependent on the contribution of groundwater relative to downgradient alluvial flow in the stream. Chloride concentrations, [Cl - ], shifted from decreasing to increasing, in both reaches during the eight week period, and were generally higher downgradient. The decreasing [Cl - ] trend suggests that high Spring flows dissolve, and transport stored chloride downstream, while the increasing [Cl - ] trend, suggests that during low Summer flows, evapotranspiration concentrates chloride- in the stream water. Nitrate (NO ₃ ⁻ ) results indicated that the beaver meadow was a source of nitrate at low flows and suggests nitrate retention varies seasonally. The study also provided evidence of enhanced water storage in the beaver meadow. The combined findings suggests that beaver activity increases late season water storage, and affects the timing and magnitude of nutrient cycling, in western semi-arid watersheds.