Beaver dam influences on streamflow hydraulic properties and thermal regimes

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.