River otters (Lontra canadensis) are important predators in aquatic ecosystems, but few studies quantify their prey consumption. We trapped crayfish monthly as an index of availability and collected otter scat for diet analysis. We measured otter daily energy expenditure (DEE) with the doubly labeled water method to develop a bioenergetics model for estimating monthly prey consumption. Crayfish (Orconectes meeki) catch-per-unit-effort was positively related to stream temperature indicating crayfish were more available during warmer months. Crayfish dominated otter diets and were present in 100% of scat samples. In contrast, fish occurrences declined from 86% in the winter and spring to 23% during the summer and autumn. Estimates of DEE averaged 4 738 kJ d-1 for an otter with a body mass of 7 842 g. Total biomass consumption ranged from 35 079 g mo-1 (wet mass) to 52 653 g mo-1 corresponding to a high proportion of fish and crayfish in the diet, respectively. Otter consumption represents a large fraction of prey production indicating potentially strong effects of otters on trophic dynamics in stream ecosystems.