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Outwash plains, sometimes known as sandar or valley trains, are broad, gently sloping, sheets of outwash sediment deposited by meltwater streams fed by glacier ablation (e.g. Church 1972; Church & Gilbert 1975; Benn & Evans 2010). They may be formed by coalescing outwash fans fed by streams emerging directly from a former ice margin or from proglacial braided rivers; such glacial meltwater streams usually have highly variable discharge and, especially when flow is high, transport large quantities of glacier-derived suspended sediment and bedload (Church & Gilbert 1975). An example of a drowned outwash plain exists off northern British Columbia in Hecate Strait (Fig. 1a), where rapid glacial retreat left an extensive plain that formerly stretched 150 km across the strait linking the British Columbia mainland to the islands of Haida Gwaii. Fig. 1. Multibeam bathymetry and cross-profile of an outwash plain that abruptly ends at a …