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Graduate Defense: Luise Winslow

October 22 @ 2:00 pm - 3:00 pm MDT

Thesis Information

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

Program: Master of Science in Civil Engineering

Advisor: Dr. Kevin R. Roche, Civil Engineering

Committee Members: Dr. Anna Bergstrom, Geosciences, and Dr. Mojtaba Sadegh, Civil Engineering

Abstract

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 function. 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 (Mg2+) concentrations decreased in both reaches, during the eight week period, and suggests Mg2+ concentrations were dependent on the contribution of groundwater relative to downgradient alluvial flow in the stream. Chloride (Cl-) concentrations 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 Cl- in the stream water. Nitrate (NO₃⁻) results indicated that the beaver meadow was a source of (NO₃⁻) at low flows and suggests nitrate retention varies seasonally. The combined findings imply that beaver activity stabilizes seasonal groundwater contributions to streams, by increasing aquifer recharge and late season storage.