Riparian areas are heavily impacted by dams and, despite their high biodiversity and ecological importance, continue to be threatened. Over half of the world’s largest rivers are dammed, with over two million dams in the United States alone. Little is known about the effects of damming on riparian vegetation. Because dams change flow regimes, sediment supply, and seed transport, one might expect downstream reaches to have lower diversity. In 2005 we observed reduced vascular plant diversity in riparian zones below 64 m high Glines Canyon Dam and 33 m high Elwha Dam along the Elwha River, Olympic National Park, Washington; however, extreme flood events have since then altered the system. To test whether the observed reduction in diversity persists, we studied the effects of these two dams again in 2010. In the summer of 2005, 120 100m2 plots were established across geomorphic landforms along 15 transects, located in free-flowing river reaches above, below, and between the dams. Within each plot riparian vegetation, species composition, and percent cover was sampled. In the summer of 2010, 63 of these plots were resampled in addition to 26 newly established plots. Average species richness per 100 m2 plot was compared among reaches and time periods.
From 2005 to 2010, there was a 26% increase in species richness (p=0.03) in the lower reach; however there were no significant changes in the other reaches. In both 2010 and 2005 there was a significant decrease in native species richness (26% and 30% respectively) from the furthest upstream to furthest downstream reach (p<0.001 for both). Exotic species richness was significantly higher in the downstream reach compared to the upstream reach both years (p<0.01 for both). These results suggest that dams may negatively impact native species diversity in the downstream reach, but other possible causes cannot be excluded. Further, this study shows that the pattern of decreased diversity we observed downstream from dams in 2005 continues to persist, despite large flood events in the intervening years. The removal of the two dams on the Elwha will begin in 2011; understanding pre-dam removal vegetation patterns over multiple years will provide valuable baseline data to assess the effects of dam removal on riparian vegetation.