We used deuterium to quantify the contribution of allochthonous detritus to gizzard shad diets across 11 reservoirs in Ohio, USA with land use ranging from 11 to 91% agricultural. Deuterium signals for phytoplankton varied among reservoirs (-252.2‰ to -152.1‰) but all were depleted relative to terrestrial vegetation (-111.6‰). Using a modified two-source mixing model, we found that the degree of allochthony varied among lakes of contrasting productivity and land use. Even in hypereutrophic Acton Lake, gizzard shad diets were nearly 20% terrestrial. Our results suggest that terrestrial detritus subsidizes gizzard shad populations, thereby contributing to the strong effects of this fish on reservoir ecosystems.