Impacts of gravel jetting on the composition of fish spawning substrates: Implications for river restoration and fisheries management

Fine sediments can impact river biota, with egg and larval stages of lithophilic fish particularly sensitive to deposition of sand- to clay-sized particles ('fines') in spawning gravels. Mitigation and restoration methods include jetting to cleanse gravels of fines. Despite wide application, impacts of jetting on gravel composition and quality have rarely been quantified. Here, gravel jetting impacts on sediment composition in the River Great Ouse (UK), were tested during an in-situ experiment completed at riffle (55.6 ± 13.4 m2) and patch (0.3 m2) scales to determine its magnitude and persistence on surface and subsurface substrate conditions. Before-after (riffle) and control-impact (patch) designs were used, with bedload sediment traps installed downstream of experimental patches to investigate the sediments mobilised during jetting. At the riffle scale, surface grain size was significantly altered; fines were removed resulting in coarser and better-sorted sediments. Similar patterns were detected at the patch scale, although sediment sorting was not significantly altered. Despite reduced fine sediment content of subsurface gravels at the riffle scale, the overall grain size composition was not significantly altered. At the patch scale, no subsurface improvements were detected. Temporally, at the riffle scale, no changes in surface or subsurface sediments lasted more than 12 months; patch scale changes generally persisted for less than 3 months. Thus, whilst gravel jetting could improve spawning gravel quality for surface spawning fishes, including European barbel Barbus barbus, its effects are short-lived. Because subsurface sediments are not affected by gravel jetting, the benefits are limited for redd-building fishes, such as salmonids. Consequently, reducing fine sediment delivery to rivers, such as by changes in agricultural practices, is more sustainable for managing excessive river sedimentation.