Growing urban environments stress hydrologic systems and impact downstream water quality. We examined a third-order catchment that transitions from an undisturbed mountain environment into urban Salt Lake City, Utah. We performed synoptic surveys during a range of seasonal baseflow conditions and utilized multiple lines of evidence to identify mechanisms by which urbanization impacts water quality. Surface water chemistry did not change appreciably until several kilometers into the urban environment, where concentrations of solutes such as chloride and nitrate increase quickly in a gaining reach. Groundwater springs discharging in this gaining system demonstrate the role of contaminated baseflow from an aquifer in driving stream chemistry. Hydrometric and hydrochemical observations were used to estimate that the aquifer contains approximately 18% water sourced from the urban area. The carbon and nitrogen dynamics indicated the urban aquifer also serves as a biogeochemical reactor. The evidence of surface water-groundwater exchange on a spatial scale of kilometers and time scale of months to years suggests a need to evolve the hydrologic model of anthropogenic impacts to urban water quality to include exchange with the subsurface. This has implications on the space and time scales of water quality mitigation efforts.

Climate change influences on mountain hydrology are uncertain but likely to be mediated by variability in subsurface hydrologic residence times and flow paths. The heterogeneity of karst aquifers adds complexity in assessing the resiliency of these water sources to perturbation, suggesting a clear need to quantify contributions from and losses to these aquifers. Here we develop a stream centric method that combines mass and flow balances to quantify net and gross gains and losses at different spatial scales. We then extend these methods to differentiate between karst conduit and matrix contributions from the aquifer. In the Logan River watershed in Northern Utah we found significant amounts of the river water repeatedly gained and then lost through a 35‐km study reach. Further, the direction and amount of water exchanged varied over space, time, and discharge. Streamflow was dominated by discharge of karst conduit groundwater after spring runoff with increasing, yet still small, fractions of matrix water later in the summer. These findings were combined with geologic information, prior subsurface dye tracing, and chemical sampling to provide additional lines of evidence that repeated groundwater exchanges are likely occurring and river flows are highly dependent on karst aquifer recharge and discharge. Given the large population dependent on karst aquifers throughout the world, there is a continued need to develop simple methods, like those presented here, for determining the resiliency of karst groundwater resources.

Beaver dams have significant impacts on the hydrology, temperature, biogeochemical processes, and geomorphology of streams and riparian areas. They have also been used as a viable tool in restoring impaired riverine systems. Because of the dynamic nature of beaver dams, these influences vary and are difficult to quantify. To begin understanding the impacts of beaver dams in mountain streams, we developed 1D hydraulic models for a beaver impacted reach that includes eight dams and a non‐impacted reach to compare hydraulic responses (e.g. channel depth, width, and velocity distributions). We also compared observations of substrate size distributions for different geomorphic/habitat units within each reach. Results from the models indicated shifts in channel hydraulics through statistically significant increases in depths and widths as well as a decrease in flow velocities through the beaver impacted reach. These hydraulic adjustments, as a result of beaver dams, are consistent with observed changes in the increased variability and spatial heterogeneity in sediment size distributions. Through the application of three different modelling approaches, we found that a relatively low number of beaver dams would result in significant changes in channel hydraulics. These results provide preliminary information regarding the number of dams per unit stream length required to begin meeting various restoration goals.

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