Sediment removal involves removing gravel or sand from the tops of bars or from the riverbed, to increase channel capacity and lower water levels in the channel. Dredging is common in canals, ditches and some smaller rivers.
The Fraser River deposits large amounts of sediment. Gravel (mixed with sand) is deposited on the riverbed in the gravel reach, which extends from Hope downstream to Sumas Mountain. Downstream of Sumas Mountain, finer sands are deposited in the sand reach. Both the gravel and sand reaches of the lower Fraser River provide critical spawning and rearing habitat for salmon, sturgeon and other fish.
Concerns have been raised that gravel accumulation in the lower Fraser River gravel reach may increase flood levels, increase riverbank erosion, and limit navigation. Sediment removal is suggested as a means to reduce these hazards.
There is limited evidence on the effectiveness of sediment removal activities on reducing overall Fraser River water levels during a flood. One modelled scenario estimates that removing two million cubic metres of sediment from the tops of all gravel bars near Chilliwack would lower flood levels by about 12 cm. Large-scale sediment extraction could potentially destabilize the river channel, with the risk of adverse consequences for shorelines, dikes and fish habitat. Any long-term removal program would require regular monitoring and modelling to understand the river’s sediment budget, effects of sediment removal on flood levels and erosion patterns, as well as ecosystem impacts.