Twelve mile creek channel morphology, substrate composition, and the macroinvertebrate community changes resulting from multiple dam removals.

摘要

Dam removal is gaining global attention as a potential restoration tool for impacted rivers. As the practice of dam removal increases, physical, chemical and biological monitoring before and after dam removal is essential to our understanding of the ecological consequences of this type of disturbance. Under the Lake Hartwell Restoration and Compensation Determination Plan, two dams (>9 m height) were removed from the Twelve Mile Creek watershed (170 km2) in Pickens county, South Carolina. To assess the short term impacts of dam removal on the Twelve Mile Creek ecosystem we conducted biomonitoring of macroinvertebrate species and surveys of river substrate and channel geomorphology in Fall 2006 prior to dam removal and again following dam removal in Fall 2011and Fall 2012. Prior to dam removal, the impounded reaches above each dam had low levels of species abundance and diversity indicative of the impacts of dams on the macroinvertebrate assemblage (Poff and Hart 2002). The above dam sites were characterized by shallow channel depths, lower velocities and sandy substrates with very few sensitive species present. In contrast, the below dam sites had become stable, lotic habitats having had adjusted to a century of impoundment and demonstrated high richness and diversity with a higher percentage of intolerant species and predators. Following dam removal, analysis of the macroinvertebrate community assemblage resulted in detectable differences in habitat condition based on observed increases or decreases in the biological response variables of total number of taxa (TN), taxa richness(TR), Epemeroptera,Plecoptera,and Trichoptera richness(EPTR), percent gatherer-collector(GC),percent filterer-collector(FC), and percent predator as well as changes in the substrate particle size and distribution (D50 mm), depth (m), and velocity (m/sec). Results suggest a decrease in macroinvertebrate species that are sensitive to the effects of sedimentation and an increase in the embededness of the substrate immediately following dam removal in 2011. The need for more long-term assessments of dam removal and river restoration practices is demonstrated by the difficulty of predicting how long a river will take to recover and lack of understanding of the ecological response aquatic ecosystems have to a dam removal disturbance.