Beaver Versus Human: the Big Differences in Small Dams

As beavers (Castor spp.) are reintroduced to streams in the northern hemisphere and nature-based and process-based river restoration emphasizes human-built analogues for beaver dams, it is reasonable to ask how beaver dams and ponds compare to different types of small dams constructed by humans for diverse purposes. We use published research as a basis for comparing the effects of beaver dams and four types of human dams-beaver dam analogues, earthen embankments perpendicular to flow that are used to create stock ponds, check dams, and mill dams. We compare these dams with respect to water balance and three-dimensional hydrologic connectivity, sediment, particulate organic matter and carbon, nutrients, habitat, and biota. These assessments inform us in ranking small dams from generally most beneficial with respect to providing ecosystem services (beaver meadows with multiple dams) to least beneficial (mill dams), recognizing that beaver meadows may not be beneficial at some sites for infrastructure and human property within the river corridor, and mill dams may be beneficial at some sites where they impede upstream migration by invasive species or provide lentic habitat in the river corridor. This comparison among dam types highlights the need for research on the cumulative effects of multiple small dams along a river corridor and across a river catchment, as well as identifying gaps in our understanding of the effects of specific types of dams, including the effects of stock ponds and check dams on carbon and nutrient dynamics.