Many processes contribute to nutrient transfer from terrestrial to aquatic systems, but in most cases the contribution of particular organisms is unknown. In this study, we explore how a Bradford-reactive soil protein (BRSP) produced by arbuscular mycorrhizal fungi may provide nutrients to river ecosystems. Along a floodplain in Montana, we extracted BRSP from soils and related the protein concentrations to the age of soil surfaces. We identified BRSP in surface soils, as well as to a depth of 1.4 m, and found that the protein accumulates through time. We also detected BRSP in foam from five rivers in the western United States. Experiments were conducted that demonstrate that the protein may be leached or washed from soils and become a constituent of foam when mixed into turbulent water. We propose that terrestrially derived soil protein may enter rivers via erosion and leaching and serve as a nutrient source for aquatic organisms.