Physical and ecological response to disturbance by gravel mining in a large alluvial river

Abstract

The role of sediment transport during high flows for restoring fish habitat was demonstrated following an experimental gravel removal from Fraser River, British Columbia, Canada. Dry bar scalping 69 000 m3 of river sediment left a topographically simple removal area with a loose surface of gravel and sand. Two subsequent, below-average floods yielded no gravel replenishment but restored substrate grain size and some topographical complexity at the habitat scale. A third, above-average flood replenished 31% of the removal volume. High-elevation bar area, which provides fish habitat at high flows, remained 25% smaller after the three floods. Effects of mining on the fish community could not be confirmed. Benthic invertebrates recolonized the removal site immediately after mining, and differences in community composition compared with three reference sites disappeared during the first flood. Results suggest that physical changes due to this mining operation fell within the range to which local aquatic populations are accustomed during flooding, because the ecological response was modest and short-lived. Despite an extensive sampling program, inherent variability in the biological data reduced statistical power to detect an effect. Monitoring programs to support adaptive management of river fisheries will require substantial investment and planning to yield definitive results.