Fencl, J. S., M. E. Mather, K. Costigan, and M. D, Daniels. 2015. How Big of an Effect Do Small Dams Have?; Using Geomorphological Footprints to Quantify Spatial Impact of Low-Head Dams and Identify Patterns of Across-Dam Variation PLoS ONE 10(11): e0141210. doi:10.1371/journal.pone.0141210
Abstract Longitudinal connectivity is a fundamental feature of streams and rivers that can be broken by dams. Over 2,000,000 low-head dams (<6 m high) fragment United States streams and rivers and can alter biodiversity. Despite potential adverse impacts of this ubiquitous disturbance, the spatial impact of low-head dams is largely unknown. Progress for research and conservation is impaired by not knowing how low-head dams affect natural systems or the magnitude of their impact. Based on the geomorphic literature, we refined a methodology that allowed us to quantify the spatial impact of low-head dam impacts (i.e., dam footprint), assess variation in dam footprints across individual low-head dams within a single watershed, and identify select aspects of the ecological context of this variation. We quantified width, depth, and substrate profiles upstream and downstream of six low-head dams within the Upper Neosho River subdrainage, Kansas. Based on changes in substrate size, dam footprints averaged 6.7 km upstream (range 2.2 – 13.7), 1.2 km downstream (range 0.2 to 1.6), and 7.9 km total (3-15.3) footprint per dam. Together, these alterations disturb 47.3 km of the Upper Neosho River subdrainage. Despite differences in size, location, and original function, the geomorphic footprints of the six low-head dams in the upper Neosho drainage were relatively similar. The number of upstream dams and proximity to upstream dams, but not dam height, affected the spatial extent of dam footprints. In summary, ubiquitous low-head dams can individually and cumulatively alter lotic ecosystems. Both characteristics of individual dams and the ecological context of neighboring dams affect low-head dam impacts within and across watersheds. For these reasons, low-head dams require a different approach for research and management than the individualistic approach that has been applied to larger dams.