Performance of bottom ramps to mitigate gravel habitat bottlenecks in a channelized lowland river

The long-term performance of measures to restore in-stream habitat in gravel bed rivers is uncertain in the presence of impoundments, land use pressures, and fine sediment inputs. The goal of this study was to evaluate the longer-term performance of five bottom ramps, designed to facilitate fish passage, and constructed similarly to artificial riffles to provide compensatory gravel riverbed habitat for benthic invertebrates and lithophilic, coarse-substrate-preferring fish in a channelized lowland river. Bottom ramp age did not significantly influence habitat conditions indicated by a lack of correlations with the percentage of fine sediment less than 2 mm, the organic content of the substrate, and the years since construction. A significant decrease in the relative abundances of coarse-substrate-preferring benthic invertebrates corresponding to project construction age was found, but there were no significant differences in the density of rheophilic, fast-flow-preferring or lithophilic fish species among sites. This study presents substantial evidence that similarly constructed bottom ramps in comparable environmental settings provided sufficient habitat for sensitive benthic invertebrates and fish to be present for over 13 years. However, a sudden decrease in habitat suitability cannot be excluded in the long-term if there is a fine sediment deposition threshold, which results in ramps becoming full. Nevertheless, bottom ramps are recommended as effective measures to enhance longitudinal connectivity, fish passage, and gravel habitat provision in channelized lowland rivers.