Beaver dam influences on streamflow hydraulic properties and thermal regimes

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