Influence of small dams on Alabama freshwater mollusk populations

Impoundments are widely recognized as having dramatic impacts on freshwater biota.  However, our understanding of how large dams affect mollusks greatly surpasses our understanding of the effects of smaller structures.  From 2006-2008 we assessed the impacts of 22 low-head dams (i.e., those <10 m height) on lotic habitats and mollusks in Alabama streams.  We measured bivalve and gastropod abundance using a combination of quadrats and timed searches.  We surveyed three reaches immediately downstream of each dam and in reference reaches located both up-and-downstream.  Streams with intact small dams supported more abundant mussel populations than did streams with breached (>50% of channel impeded) or relict (<50% channel impeded) dams.  Further, mussel species richness was low in streams with breached dams but was much greater in streams with relict dams.  In contrast, Elimia spp. densities were higher in streams with breached dams.  We speculate that long-term streambed stability associated with some intact small dams may benefit mussel populations.  In contrast, Elimia spp. and other gastropods appear more sensitive to local-scale physicochemical conditions, including flow velocity and substrate composition.  Although removal of breached dams should take priority over removing intact structures, a case-by-case approach is necessary to evaluate impacts to sensitive mollusk taxa.