Stream fragmentation by road culverts

Rivers are defined by the connections between lower and higher order reaches. Roads crossing rivers can cause fragmentation by altering hydrodynamics, changing stream geomorphology, and creating barriers to animal dispersal. We hypothesized that culverts (pipes) used at road crossings impede crayfish movement and alter stream benthic characteristics. We tested this hypothesis by characterizing crayfish populations and stream characteristics upstream and downstream of culverts in streams in northern, lower Michigan. We estimated crayfish densities using sardine-baited minnow traps (27 locations). At 9 sites (6 culverts, 2 bridges, and 1 roadless control) we measured DO, flow, and substrate type at 3 upstream and 3 downstream cross-sections (10 measurements/cross-section). Crayfish catch per unit effort (CPUE) differed among sites and upstream versus downstream of road crossings. Downstream sites did not show consistently higher crayfish densities. Upstream and downstream reaches near culverts differed in characteristics however differences remained inconsistent among sites. Geomorphic features (catchment area and slope), stochastic events (beaver dams) and culvert design (size) appear to influence culvert effects on streams and crayfish populations. Culverts should only be used to cross low gradient streams with small catchments to minimize their effects on crayfish and stream benthic habitats.