Vegetation community response to hydrologic and geomorphic changes following dam removal

Dam removal can restore fish passage, natural flow regimes, sediment transport in streams, dispersal of organic matter, and drift of aquatic insects. However, dam removal also impacts the riparian vegetation, with both immediate and delayed responses. In this study, we measure vegetation change at the Merrimack Village Dam site on the Souhegan River in Merrimack, NH, USA. The August 2008 removal caused a similar to 3-m drop in water level and rapid erosion of impounded sediment, with similar to 50% removed in the first 3months. Terrace, floodplain, and wetland communities were surveyed in summer 2007, 2009, 2014, and 2015. Temporal change was quantified using Analysis of Similarity on the Bray-Curtis dissimilarity matrix. Only herbaceous vegetation closest to the river channel and in the off-channel wetland changed significantly. The herbaceous plots directly adjacent to the impoundment eroded to bare sand in 2009, but by 2014, the original riparian fringe community had re-established in the newly developed floodplain. Between 2007 and 2014, the off-channel wetland area changed from aquatic species to a stable terrestrial community that persisted without significant change in 2015. The vegetation response was greatest in areas with the largest geomorphic and hydrologic change. These included the channel margin where erosion and bank slumping created an unstable scarp. The mid-channel island and off-channel wetland were strongly affected by the lowered water table. However, large unvegetated areas never persisted nor did the areal coverage of invasive species expand, which are two frequent concerns of dam removal stakeholders.