Fluvial processes and facies sequences in the sandy braided South Saskatchewan River, Canada
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ABSTRACTThe South Saskatchewan River has a long term average discharge of 275 m3/sec, with flood peaks in the range of 1500 to 3800 m3/sec. South of Saskatoon, the four major types of geomorphological elements recognised are channels, slipface‐bounded bars, sand flats and vegetated islands and floodplains. Major channels are 3‐5 m deep, up to 200 m wide, and flow around sand flats which are 50‐2000 m long, and around vegetated islands up to 1 km long. At areas of flow expansion, long straight‐crested cross‐channel bars form. During falling stage, a small part of the crest of the cross‐channel bar may become emergent, and act as a nucleus for downstream and lateral growth of a new sand flat.The dominant channel bedforms are dunes, which deposit trough cross bedding. Cross‐channel bars deposit large sets of planar tabular cross bedding. Sand flats that grow from a nucleus on a cross‐channel bar are mostly composed of smaller planar tabular sets, with some parallel lamination, trough cross‐bedding, and ripple cross‐lamination. A typical facies sequence related to sand flat growth would consist of in‐channel trough cross‐bedding, overlain by a large (1‐2 m) planar tabular set (cross‐channel bar), overlain in turn by a complex association mostly of small planar tabular cross‐beds, trough cross‐beds and ripple cross‐lamination.By contrast, a second stratigraphic sequence can be proposed, related only to channel aggradation. It would consist dominantly of trough cross‐beds, decreasing in scale upward, and possible interrupted by isolated sets of planar tabular cross‐bedding if a cross‐channel bar formed, but failed to grow into a sand flat. During final filling of the channel, ripple cross‐lamination and thin clay layers may be deposited. In the S. Saskatchewan, these sequences are a minimum of 5 m thick, and are overlain by 0.5‐1 m of silty and muddy vertical accretion deposits.
