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

Abstract

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.

Structural microhabitat use by endemic cyprinids in a Mediterranean‐type river: Implications for restoration practices

“…This could be done through the recruitment (by allowing passive natural input) or placement (active introduction) of large pieces of wood from natural riparian vegetation within the river channel, which would provide an additional form of cover, thereby increasing the habitat complexity for fish (Howell et al, 2012). Gravel bedforms and artificial riffles to benefit lithophilic spawners, such as barbel and nase, could be considered in future restoration plans, as they have been shown to enhance the variability in river depth and flow heterogeneity, and to offer coarse substrata that improve spawning habitat (Goeller & Wolter, 2015). Depth, velocity and substratum were also key variables for barbel and nase, particularly the latter, which displayed shifts to fasterflowing positions with coarser substrata during the spring.…”

“…Depth, velocity and substratum were also key variables for barbel and nase, particularly the latter, which displayed shifts to fasterflowing positions with coarser substrata during the spring. Gravel bedforms and artificial riffles to benefit lithophilic spawners, such as barbel and nase, could be considered in future restoration plans, as they have been shown to enhance the variability in river depth and flow heterogeneity, and to offer coarse substrata that improve spawning habitat (Goeller & Wolter, 2015). These substrata (particularly larger pebbles and boulders) should ideally be placed below the water surface to prevent the rehabilitated section turning into a feeding area for fish-eating birds, such as herons (Cowx & Welcomme, 1998).…”

Model-Based Evaluation of Urban River Restoration: Conflicts between Sensitive Fish Species and Recreational Users

“…River-bottom ramps result from the removal of linear low weirs and have honeycomb-shaped structures constructed with natural large rocks to disperse the hydraulic head over a short distance but with a gentle slope. These provide a high diversity of microhabitats with different velocities, depths, and substrata, e.g., for sensitive benthic invertebrates and fish in gravel bar rivers [89]. However, although slight improvements were found, the functionality of the created habitats remains to be proven for the nase.…”