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Abstract

Bella Coola River drains 5050 km2 of glacierized mountains on the central coast of British Columbia. Spring snowmelt and autumn rainstorm floods occur up to 1000 m3s The valley-fill is composed of Quaternary glacial and fluvial sediments, with modern alluvium representing less than 3 percent of the total volume. Proportions of light and heavy minerals in alluvial deposits indicate that headwater volcanic terrain is the dominant sediment source; however, distal tributaries draining plutonic rocks are locally important.

Sedimentation within the modern channel yields medial, lateral and point bars and vegetated channel islands. Avulsions within ‘sedimentation zones’ result in a network of slough channels. Floodplain development occurs in three ways: infilling of sloughs after channel avulsions, lateral accretion of channel bars, and overbank deposition. The most prominent facies assemblage is horizontally and trough cross-stratified sands over massive gravels. Intervening stable reaches are cobble-paved zones through which sediment is transported directly. Facies within these reaches comprise horizontally stratified sands and silts, indicating that floodplain development also occurs by vertical accretion. This association of channel zone and floodplain facies is widespread in mountain valley rivers in the northern Cordillera today. They are not aggrading and would not be prominent in the stratigraphic record.

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