Abstract

The alluvial architecture and evolution of two kilometer-scale compound bars in the wandering gravel-bed Fraser and Squamish rivers, British Columbia, Canada, are described. Integrating ground-penetrating radar, bathymetry, and aerial photographs enables the internal architecture to be linked to the evolution of the gravelly barforms over the previous 50 years. These linkages reveal the sedimentary mechanisms that formed the various architectural packages within compound bars and unit bars. Growth of compound bars is controlled by the accretion of unit bars onto discrete segments along their gravelly edges. The attachment of unit bars deflects the thalweg to impinge on and erode specific portions of bars and channel banks. Bar growth leads to the stabilization of bars, vegetation colonization of bar interiors, and island formation. It is the formation of islands, along with channel avulsions, that maintains channel division in wandering rivers.

Seven styles of gravelly deposition are imaged in the alluvial architecture. Vertical-accretion deposits formed from the deposition of gravelly bedload sheets are the most common strata. A moderate abundance of slipface deposits preserves high-relief bar margins. Lateral accretion dominates point-bar deposits. Downstream-accretion deposits govern some phases of down-bar growth. Partial and complete channel-fill and chute-fill deposits are eroded into underlying sediments, as are scour-and-fill deposits. Upstream-accretion deposits are uncommon.

A depositional model of gravelly channel bars in the two rivers is presented and reveals that the architecture is made up of depositional styles similar to those observed in braiding-river successions, although the sedimentary packages are preserved in different proportions. Differences in braiding-river and wandering-river sedimentology largely reflect the relatively frequent migration of channels and bars in braiding rivers, which preserve high proportions of channel fill and chute fill, and confluence scour-and-fill deposits. Conversely, the moderately stable island and channel network in wandering rivers limits channel shifting and consequently preserves a low number of channel fills. Moderate proportions of slipface strata and coherent patterns of sand deposition along bar tops provide evidence of comparatively uniform flow patterns in independent channel segments divided by islands. These patterns of bar sedimentation and channel shifting preserved in the alluvial architecture appear to be signature characteristics of wandering rivers. The occurrence of similar architecture in both the Fraser and Squamish rivers suggests that the model likely applies to most wandering gravel-bed rivers.

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