Beaver Library

Expansion of populations of beaver Castor canadensis in northern Wisconsin has raised concerns over warming of coldwater fish habitats as a result of impoundments created by the mammals. We examined temperature with a network of electronic thermographs that recorded hourly water, air, and soil temperatures on four headwater streams occupied by beaver during summer 1990 and 1991. Stream temperatures followed air temperatures, even near groundwater sources. There was no consistent relationship between size or number of beaver impoundments and the degree of downstream warming. Large impoundments, although often warming downstream temperatures slightly, dampened temperature fluctuations immediately downstream. Local groundwater inflow and vegetative and topographic shading also dampened warming by impoundments. Several beaver impoundments were removed to evaluate ensuing temperature changes. Removal of beaver dams did not generally reduce the difference between upstream and downstream temperatures; in some cases dam removal increased the warming rate. Direct thermal benefits of dam removal in headwater streams may be outweighed by the potentially disruptive effects on the composition of fish and invertebrate communities downstream. It is suggested that management focus on relating topographical and geographical attributes to the potential for substantial groundwater discharge and to suitable summer temperatures for coldwater species such as brook trout Salvelinus fontinalis.