CHANNEL AND FLOODPLAIN FACIES IN A WANDERING GRAVEL-BED RIVER

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 m3sp'. 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. km2) but contributes less than 25 percent of the discharge. The Atnarko headwater is an uplifted plateau containing numerous lakes along principal drainage lines. The high storage capacity of the lakes severely limits sediment de- livery to the Bella Coola River which, in comparison with the Talchako River, might be no more than 5 percent of the total load at their confluence. The southern boundary of the basin includes some of the highest areas in the Coast Mountains (to +3035 m). The entire basin was heavily gla- ciated in Pleistocene time and a late neoglacial resurgence of alpine glaciers ended at the turn of the twentieth century. The Talchako basin remains heavily glacierized over 25 percent of its area. Upland surficial materials in Bella Coola basin consist of till, kames, colluvium and local pockets of alluvium. Approximately 22 percent of the basin is exposed bedrock. Orographic effects over the Coast Mountains produce a strong precipitation gradient within the basin. In the west- ern section of the catchment annual precipitation exceeds 2000 mm and declines to less than 500 mm in the east: Runoff to Bella Coola River is dominated by two processes: spring snowmelt and fall rainstorms. The largest flows (maximum mean daily Q > 1000 m's-') occur in the au- tumn with the incidence of heavy rains and melting of re- cently fallen snow. Spring snowmelt floods are usually smaller. Bella Coola River is a cobble-gravel stream with gra- dients between 0.0010 and 0.0031. Single thread and mul- tiple channels occur in both sinuous and non-sinuous reaches of the river. Contemporary alluvial processes along the Bella Coola are controlled by sediments which have accumulated within the valley during the late glacial and Holocene ep- ochs, and by the distribution of upland sediment sources. A brief description is provided of the origin and signifi- cance of the older sediments, and the influence of upland sources is assessed. Details of sediment deposition, facies development and floodplain evolution are then given. SEDIMENTOLOGICAL CONTEXT