Adult aquatic insects are a globally important subsidy in terrestrial food webs. However, our understanding of their importance is largely limited to studies that measure predation of live insects by terrestrial predators. Yet the flux of adult aquatic insects to terrestrial detrital pools may also be an important subsidy pathway, particularly in cases where insect production exceeds the consumption capacity of predators. We used empirical measures of giant salmonfly (Pteronarcys californica) emergence from 37 sites to model potential detrital deposition in nearshore riparian soil food webs. Typically, giant salmonflies emerge en masse for one week each year, and can be locally superabundant. Median detrital deposition by salmonflies ranged between 0.4 and 0.7 gC, 0.04 to 0.09 gN, and 0.002 to 0.005 gP/m²/yr, depending on whether 25% or 100% of available salmonflies entered detrital pools. For a small number of sites with large salmonfly populations, deposition equaled or exceeded annual secondary production of terrestrial insects, annual atmospheric N deposition, and annual atmospheric P deposition. The fact that these values rival yearly nutrient budgets is particularly striking because giant salmonfly deposition represents a subsidy from a single species emerging over a single week. The consequences of this deposition in terrestrial food webs are largely unknown, but it is likely that salmonflies can have important effects on nearshore soil nutrient budgets similar in magnitude to those of other important ecosystem processes.