• Floodplain;
  • large-scale rotational failures;
  • mud-dominated river;
  • oblique accretion deposits;
  • overbank deposits;
  • silt alluvium

Abstract The Red River, Manitoba, is a mud-dominated, meandering stream that occupies a shallow valley eroded into a clay plain. The valley-bottom alluvium is the product of incision and lateral migration of river meanders. As revealed by a transect of five boreholes located across the floodplain at each of two successive river meanders, the alluvial deposits range from about 15 to 22 m thick and are composed primarily of silt. Sedimentary structures in the cores are weakly defined and consist mostly of beds of massive silt, thick (>0·4 m) massive silt and disturbed silt. Interlaminated sand and silt, and sand beds form relatively minor deposits, principally within the lower half of the alluvium, and thin beds of medium-coarse sand and pea gravel can be present locally within the lower metre of the alluvium. The alluvium is interpreted to consist of overbank deposits from 0 to 2–3 m depth, oblique accretion deposits from 2–3 to 8–12 m depth and oblique accretion and/or channel deposits from 8–12 m to the base of the sequence. The massive bedding within the oblique accretion deposits is interpreted to represent the remnants of couplet deposits that were initially composed of interbedded, muddy silt and sand-sized silt aggregates, as is consistent with the contemporary bank sedimentation. The post-depositional disintegration and/or compaction of the aggregates has caused the loss of the sand-sized texture. The disturbed silt beds are interpreted as slump structures caused by large-scale rotational failures along the convex banks. Overall, the Red River represents a portion of a continuum of muddy, fine-grained streams; where the alluvium lacks a distinct coarse unit, oblique accretion deposits form a majority of the floodplain, and large-scale slump features are present.