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 temperature variability within a beaver dam complex reached up to 10.5 °C due to the diversity of geomorphic units. At the geomorphic unit scale, the highly altered flow velocity and depth distributions within primary geomorphic units provide an explanation of the temperature variability within the dam complex and insight regarding increases in habitat heterogeneity.
Highlights
The presence of beaver dams in streams increases variability in channel hydraulics with general decreases in flow velocities and increases in flow depths (Green and Westbrook, 2009; Nyssen et al, 2011; Westbrook et al, 2006)
Few studies have investigated the influences of beaver dams on the connections between channel hydraulics and the geomorphic template (e.g., Green and Westbrook, 2009; Levine and Meyer, 2014; Pollock et al, 2007; Stout et al, 2017; Wheaton et al, 2004)
Beaver dam complex scale which includes a beaver dam or a series of beaver dams that are close to each other, the beaver pond(s), a portion of the upstream channel, and a portion of the downstream channel, and 3. geomorphic unit scale that includes geomorphic units created within the beaver dam complex that are directly related to beaver activity
Summary
The presence of beaver dams in streams increases variability in channel hydraulics with general decreases in flow velocities and increases in flow depths (Green and Westbrook, 2009; Nyssen et al, 2011; Westbrook et al, 2006). Water temperature is primarily dictated by climatic drivers (such as solar radiation, air temperature and wind speed), channel structure and complexity, groundwater influences, and riparian vegetation (e.g., Sinokrot and Stefan, 1993; Webb et al, 2008) It can dictate the distribution of aquatic species (Isaak et al, 2018). Majerova et al (2015) highlighted the importance of spatial, as well as temporal scales, when examining the influences of beaver dams on temperature They illustrated the role of individual beaver dams on cumulative downstream warming and/or cooling and demonstrated increased thermal variability after beaver colonization within a 750 m reach with multiple beaver dam complexes. Literature regarding the impacts of beaver dams on stream temperature in relation to fish are inconsistent and few studies are based on in-situ measurements (Gibson and Olden, 2014; Kemp et al, 2012)
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