Date of Award
Doctor of Philosophy (PhD)
Dr. R. G. Walker
In the study area, the South Saskatchewan River has a sandy bed (mean diameter .3 mm) with irregularly-shaped braid bars termed sand flats. These range in length from 50 to 2000. The river has an average discharge of 220 m³/sec, with a mean annual flood of 1450 m³/sec. The river has been dammed upstream of the study area since 1965, but little downcutting has occurred.
Ripples, sand waves and dunes are the equilibrium bedforms present. Ripples and dunes are well known, but sand waves are long, low bedforms with superimposed ripples, lack scour troughs, and occur at lower flow velocities than dunes. Foreset-type bars are also present, but are not equilibrium forms. They result from flow expansion around older topography. They occur at (1) channel junctions, (2) channel bends, (3) areas of channel widening, (4) places of vertical flow expansion. They deposit planar crossbeds.
Large areas of the river have many sand flats with no major channels, and may even lack minor channels. These areas are termed sand flat complexes. Where a major channel curves around a sand flat complex, a large diagonal bar is deposited. It is mainly on the tops of these bars where new sand flats form.
The major channels rarely exceed 5 m in depth, but may be 150 m wide. They are floored by sinuous-crested dunes with sand waves and ripples along their margins. The dunes build up during floods (2 m maximum amplitude). Larger dunes occur in the deeper channels.
Three different morphologies of small sand flats, symmetric, asymmetric and side, have been recognized. Each type forms from a bar which becomes partly immobilized where it become emergent. The remainder of the bar front continues to advance around this emergent nucleus. The different morphologies result because of the control exerted by preexisting deposits on the shape of the initial bar.
Larger sand flats lack these morphologies because they have been extensively modified. The major processes of modification are vertical, lateral, and upstream accretion by bars; linking of sand flats by bars; erosional action. The variable morphologies of larger sand flats reflect only their latest modification. The stratification of sand flats is mainly planar crossbed sets deposited by the bars.
During the winter, a 60 cm thick layer of ice covers the entire system. The sand flats are immobilized because their top layers of sediments are frozen. In some places, their surfaces are disrupted by fluid escape caused by high pore pressures generated by freezing. Flow proceeds down the channels under the ice. Rafting of cobbles and scouring around grounded ice blocks takes place at breakup.
The facies sequences resulting from sedimentation in the river are mainly sandy. Those which are deposited by channels consists dominantly of trough crossbeds, but lone planar crossbed sets may be present, deposited by large bars. Facies sequences which include sand flat deposits have several sets of planar crossbeds stacked on top of one another. All sequences have a zone of small crossbeds and ripple cross-lamination near the tops, resulting from shallow water deposition. They are capped by one-half metre of muddy flood-plain deposits.
Cant, Douglas J., "Braided Stream Sedimentation in the South Saskatchewan River" (1976). Open Access Dissertations and Theses. Paper 820.