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 graphic (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 graphic to graphic (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.

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