Elwha River Riparian Vegetation Response to Dams and Dam Removal

Riparian vegetation is highly diverse and influences habitat of aquatic and terrestrial wildlife. Riparian vegetation dynamics are driven by stream flow regime, and fluxes of sediment and large woody debris, all of which can be altered by river damming. Dam removal is often implemented, in part, to help restore degraded riparian vegetation by reversing the alteration of these key drivers. However, increased disturbance and sediment flux associated with transport and exposure of trapped reservoir sediment can complicate a simple return to pre-dam conditions and can favor exotic species. We are studying the effects of dams and their removal on riparian vegetation along the Elwha River in Washington State, where removal of two large dams began in September 2011. To characterize vegetation composition, structure, and diversity prior to dam removal, we sampled 60-150 vegetation plots in 2004, 2005, and 2010 along five cross-valley transects in each of three river reaches: above both dams (upper reach), between the dams (middle reach), and downstream of both dams (lower reach). In summer 2012, we resampled a subset of our plots in the lower and middle reaches to evaluate vegetation and geomorphic change. We also sampled vegetation, topography, and grain size along newly-established transects within the exposed former reservoir behind Elwha Dam, which was removed in 2011 and 2012. Plant community distribution on bottomland geomorphic surfaces along the Elwha is typical of other systems in the region. We identified 8 overstory and 26 understory communities using multivariate analyses. Young bar surfaces (5-20 yrs) were dominated by willow, red alder, and black cottonwood. Floodplains and transitional fluvial terraces (<90yrs) were generally dominated by alder and cottonwood. Mature terraces (>90yrs) were often dominated by big-leaf maple. Douglas fir occurred on both young and old floodplains and terraces. Overstory species composition was more stable from 2005 to 2010 than understory, understory species composition was more influenced by reach than overstory, and understory communities on younger landforms were less stable. From 2005 to 2010, there was a 26% increase in species richness in the lower reach but no significant changes elsewhere. In both 2005 and 2010 there was a significant decrease in native species richness (26% and 30% respectively) from the furthest upstream to furthest downstream reach. Exotic species richness was significantly higher in the downstream reach compared to the upstream reach in both years. Following dam removal, we documented different pioneer plant communities growing on different surfaces within the former reservoir behind Elwha Dam. Some surfaces were dominated by exotic species of concern to resource managers, such as Canada thistle and reed canarygrass, while others contained typical, native pioneer communities dominated by willow, alder, and cottonwood. Along our existing monitoring transects, we documented some sediment deposition on channel margins, bars and low floodplain surfaces, particularly in the lower reach; relatively little new deposition occurred in the more armored and stable middle reach. Future monitoring will reveal changes to existing vegetation communities and development of new communities in the former reservoirs as removal of the upstream dam is completed and millions of m3 of sediment are redistributed.