Sedimentation on the Western Delta-front of the Fraser River, British Columbia

Abstract

The broad patterns of sedimentation and erosion on the western delta-front of the Fraser River are described on the basis of a detailed sedimentological survey and three successive bathymetric surveys.The surveyed portion of the delta-front can be subdivided into three sedimentary environments: salt marsh, main platform, and upper fore-slope. The salt marsh lies near high tide level and is a flat to hummocky, vegetated zone about 0.5 mi (1 km) wide, having sediments with a mean grain size finer than [Formula: see text] (0.063 mm). The main platform (which includes the tidal flats below the salt marsh) is the zone that slopes gradually for about 4 mi (6 km) from the salt marsh to the break in slope at approximately 30 ft (9 m) below lowest normal tide level. It is mantled mainly with [Formula: see text] to [Formula: see text] (0.35 mm to 0.125 mm) rippled sand. The upper fore-slope has here been designated as that portion of the delta-front extending from the main platform to the limit of the bathymetric survey (− 300 ft) (− 90 m). The gradient of the upper fore-slope is variable and attains a maximum inclination of approximately 12°. Upper fore-slope sediments are muddy at, and north of the main river channel, but consist mostly of sand south of there. The sharp transition between platform sand and muddy fore-slope sediment north of the main channel suggests that the − 30 ft (− 9 m) contour there denotes the maximum depth of vigorous wave and current action.Bathymetric surveys conducted in 1968 and April and August 1972 coupled with a calculation of the gross sediment budget of the western delta-front reveal that, as a consequence probably of local physical oceanography and confinement and dredging of the main channel, yearly freshet deposition is promoting the advance of the delta-front principally off the main channel and seems sufficient, at least, to maintain the stability of the upper fore-slope north of there. However, on the southernmost segment of the western delta-front, prevailing conditions have brought about the retreat of the upper fore-slope at least during the period 1968–1972.