Abstract. Beaver dams affect hydrologic processes, channel complexity, and stream temperature in part by inundating riparian areas, influencing groundwater–surface water interactions, and changing fluvial processes within stream systems. We explored the impacts of beaver dams on hydrologic and temperature regimes at different spatial and temporal scales within a mountain stream in northern Utah over a 3-year period spanning pre- and post-beaver colonization. Using continuous stream discharge, stream temperature, synoptic tracer experiments, and groundwater elevation measurements, we documented pre-beaver conditions in the first year of the study. In the second year, we captured the initial effects of three beaver dams, while the third year included the effects of ten dams. After beaver colonization, reach-scale (~ 750 m in length) discharge observations showed a shift from slightly losing to gaining. However, at the smaller sub-reach scale (ranging from 56 to 185 m in length), the discharge gains and losses increased in variability due to more complex flow pathways with beaver dams forcing overland flow, increasing surface and subsurface storage, and increasing groundwater elevations. At the reach scale, temperatures were found to increase by 0.38 °C (3.8 %), which in part is explained by a 230 % increase in mean reach residence time. At the smallest, beaver dam scale (including upstream ponded area, beaver dam structure, and immediate downstream section), there were notable increases in the thermal heterogeneity where warmer and cooler niches were created. Through the quantification of hydrologic and thermal changes at different spatial and temporal scales, we document increased variability during post-beaver colonization and highlight the need to understand the impacts of beaver dams on stream ecosystems and their potential role in stream restoration.

Abstract. Beaver dams affect hydrologic processes, channel complexity, and stream temperature by increasing inundated areas and influencing groundwater-surface water interactions. We explored the impacts of beaver dams on hydrologic and temperature regimes at different spatial and temporal scales within a mountain stream in northern Utah over a three-year period spanning pre- and post-beaver colonization. Using continuous stream discharge, stream temperature, synoptic tracer experiments, and groundwater elevation measurements we documented pre-beaver conditions in the first year of the study. In the second year, we captured the initial effects of three beaver dams, while the third year included the effects of ten dams. After beaver colonization, reach scale discharge observations showed a shift from slightly losing to gaining. However, at the smaller sub-reach scale, the discharge gains and losses increased in variability due to more complex flow pathways with beaver dams forcing overland flow and increasing surface and subsurface storage. At the reach scale, temperatures were found to increase by 0.38 °C (3.8%), which in part is explained by a 230% increase in mean reach residence time. At the smallest, beaver dam scale, there were notable increases in the thermal heterogeneity where warmer and cooler niches were created. Through the quantification of hydrologic and thermal changes at different spatial and temporal scales, we document increased variability during post-beaver colonization and highlight the need to understand the impacts of beaver dams on stream ecosystems and their potential role in stream restoration.

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.

No abstract

Abstract. Beaver dams alter channel hydraulics which in turn change the geomorphic templates of streams. Variability 13 in geomorphic units, the building blocks of stream systems, and water temperature, critical to stream ecological 14 function, define habitat heterogeneity and availability. While prior research has shown the impact of beaver dams on 15 stream hydraulics, geomorphic template, or temperature, the connections or feedbacks between these habitat measures 16are not well understood. This has left questions regarding relationships between temperature variability at different 17 spatial scales to hydraulic properties such as flow depth and velocity that are dependent on the geomorphology. We 18 combine detailed predicted hydraulic properties, field based maps with an additional classification scheme of 19 geomorphic units, and detailed water temperature observations throughout a study reach to demonstrate the 20 relationship between these factors at different spatial scales (reach, beaver dam complexes, and geomorphic units). 21Over a three week, low flow period we found temperature to vary 2 °C between the upstream and downstream extents 22 of the reach with a net warming of 1 °C during the day and a net cooling of 0.5 °C at night. At the beaver dam complex 23 scale, net warming of 1.15 °C occurred during the day with variable cooling at night. Regardless of limited temperature 24 changes at these larger scales, the temperaure variability in a beaver dam complex reached up to 10.5 °C due to the 25 diversity of geomorphic units within the complex. At the geomorphic unit scale, the highly altered flow velocity and 26 depth distributions within primary units provide an explanation of the temperature variability within the dam complex. 27Riffles, with the greatest velocity variability and least depth variability, have the smallest temperature variability and 28 range. The lowest velocity variability occurred within margins, pools, and backwaters which exhibit the widest 29 temperature ranges, but range from shallow to deep. Overall, the predicted flow hydraulic properties for different 30 geomorphic units suggest that velocity is the primary factor in determining the variability of water temperature. 31However, water depth can also play a role as it impacts warming patterns and can dictate thermal stratification. These 32 findings begin to link key attributes of different geomorphic units to thermal variability and illustrates the value of the 33 geomorphic variability associated with the development of beaver dam complexes.