BEAVER DAM IMPACTS ON SEDIMENT AND WATER REGIME, AND IMPLICATIONS FOR RIVER RESTORATION

Fluvial discontinuities inhibit transport of water and sediment downstream, resulting in storage upstream and supply downstream of each discontinuity. Breaching of discontinuities releases stored water and sediment. River restoration, however, typically emphasizes creation of free-flowing water and sediment. In order to incorporate non-human generated discontinuities into the baseline conditions for restoration, this talk presents a hierarchical patch dynamics model of river networks, which includes patch types that vary according to the scale of observation.

Within this context, this talk presents a study of modern beaver dams in northeastern Connecticut. Three fundamental segment types— free-flowing segments, beaver ponds, and beaver meadows—are visible at the network scale (10³ m). Channel shape and sediment size data from free-flowing segments and beaver ponds at the segment scale (10² m) demonstrate four subtypes within these two groups. Segments downstream of beaver dams have multiple threads as well as coarser banks and smaller width:depth ratios, indicative of erosion. In-channel beaver dam series are highly variable, with construction of small dams storing fine sediments over short time scales, and scouring of pools upon failure. Despite obvious hydraulic and sediment regime differences, valley-wide beaver ponds and free-flowing streams have similar width:depth ratios, although valley-wide ponds are more variable than free-flowing streams.

This heterogeneity is further visible in unit scale (10¹ m) temperature data collected along 400m of streambed through several segment types. Cold groundwater enters the main channel during summer low flows at an entirely dammed tributary, at bedrock discontinuities enhanced by beaver construction, and at a spring downgradient of a 5ha beaver pond. Overall, these discontinuities increase habitat heterogeneity, critically important to river ecosystems. We recommend that river restoration include analysis of discontinuous features at the network scale, and that management of human-constructed features better match the processes of non-human generated discontinuities.