Hydrochory, the dispersal of seeds by water, is important for maintaining the diversity and genetic continuity of riparian plant communities. Dams may reduce levels of hydrochory to downstream reaches by trapping seeds within their impoundments. On Washington's Olympic Peninsula, we studied whether hydrochory in the Elwha River was affected by the Glines Canyon Dam. We also explored whether Lake Mills, the dam's impoundment, holds a hidden seed bank of trapped hydrochorous seeds, which may aid in revegetation after the dam is removed. Hydrochory levels were sampled during three time periods in July and August, 2005 using floating and submerged drift nets above and below Glines Canyon Dam. The Lake Mills seed bank was sampled along transects across Lake Mills. For all drift net samples there was a 90% reduction in seed abundance and 84% reduction in species richness below Glines Canyon Dam. The decline in seed abundance was seen at each of the three sampling times. Similar numbers of seeds were found in both floating and submerged nets, suggesting that buoyancy is not required for hydrochory in this system. Relatively few seeds germinated from the Lake Mills seed bank. There was no relationship between seed density and distance from the Lake Mills delta; however seed density declined with increasing water depth. Our results suggest that Glines Canyon Dam has reduced the rate of hydrochory in the Elwha River, which may cause fragmentation of the riparian flora and reduced diversity of riparian species below the dam.