Glacial-lake outbursts occurred in the northern Great Plains as ice-marginal lakes suddenly drained, forming an interconnected system of glacial-lake spillways. The outbursts were highly erosive, and consequently, deposits from such discharges are rare in the spillways. Glacial-lake–outburst deposits in the Souris, Des Lacs, and Moose Mountain spillways, Saskatchewan and North Dakota, have distinct bar shapes and, internally, are more massive and homogeneous than those of outwash systems. In this paper, the outburst deposits are described, and the sedimentologic and hydrodynamic characteristics of the outburst discharges are interpreted.

Coarse-grained sediment eroded from till formed large-scale bars in the spillways, as much as 2 km long and 30 m thick, both as pendant bars and bars in alcoves next to contemporaneous landslides. Internally, the bars consist of homogeneous masses of massive, matrix-supported, very poorly sorted, pebbly cobble gravel, containing boulders as much as 3 m in diameter. The maximum particle sizes in individual bars decrease downstream and toward spillway sides. The lithology of the bars resembles the local glacial drift; material from the Tertiary bedrock, in which the spillways are partially incised, is not concentrated in the bars.

Paleohydraulic calculations based on data of maximum particle size and on the Shields' entrainment function yield unreasonably large values. Supplementary calculations based on depth limits established from field observations yield realistic values; this method indicates that velocities of 2.9 to 11.7 ms−1 and discharges of 5.8 × 104 to 8.2 × 105 m3s−1 were achieved by the outburst. Sediment-water concentrations during the outburst, estimated by dividing the volume of sediment eroded by the source-lake volume, averaged ∼20% by weight. Because of this estimate, the abundant amount of fine-grained material that was eroded, and structure of the outburst deposits, we infer that outburst flows were hyperconcentrated, with sediment-water concentrations being as much as ∼40% by weight in some areas. We further infer that sediment transport was extremely efficient and that most particles, including coarse gravel, were transported in suspension. This interpretation is consistent with the sedimentology of the deposits and other conjectural evidence. Expanding sections caused coarser sediment to be deposited, indiscriminate of clast size, as poorly sorted, massive gravel bars. Finer grained sediment, sand to clay size, was transported through the spillway system into downstream glacial lakes.

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