Sedimentation in the Fraser River and its estuary, southwestern British Columbia (Canada)

Abstract

The Fraser River, the largest river (in terms of both water and sediment discharge) reaching the west coast of Canada, is a sand-dominated river in which most sediment transport occurs during freshet in late spring and early summer. More than half the sediment discharged during this 2–3 month period is sand. Throughout the rest of the year, the river is characterized by lower flow and low suspended sediment concentrations (primarily silt and clay); net offshore transport during these months is slight, and nearbottom transport appears to be landward. The dominance of sand transport in the Fraser results in an estuarine depositional regime quite different from most mud-dominated rivers and estuaries. Although most sediment in the river is carried in suspension, about 40% of the sand (20% of the total load) settles from suspension in the upper estuary and most of the rest settles prior to reaching the lower estuary. In a natural situation, much of the river sand probably would continue moving seaward as bed load, as suggested by the prevalence of migrating sand waves in the middle estuary during freshet. Longshore drift of this sand has built tidal flats that now dominate the nearshore environment. Dredging of river channels removes an appreciable part of the total annual sand load. Jetties across intertidal flats and at the river mouth have interrupted longshore transport and increased resuspension of sand in the outer estuary by channelizing flow. All of these factors should combine in shifting tidal flats and adjacent shoreline from their natural state.