Abstract

The avulsion of the Saskatchewan River (Cumberland Marshes, east-central Saskatchewan, Canada) has resulted in new floodplain deposition over an area of more than 500 km 2 since beginning in the 1870s. The avulsion deposits are predominantly fine grained and could be otherwise classifiable as "overbank" sediments in alluvial floodplains presumed to be supplied by intermittent flooding of dominant master channels. The North Angling and Cadotte channels are older pre-existing channels that were intercepted and appropriated by the expanding avulsion. Following annexation of the North Angling Channel, stable cohesive banks inhibited channel enlargement and crevassing, forcing the increasing discharges to be accommodated by overbank flow and in the process, enhancing new levee aggradation. These levee deposits coarsen upward and thin downstream as a result of greater competence and higher proximal deposition rates brought on by the larger flows. In contrast, the Cadotte Channel responded to increasing discharges by levee breaching, giving rise to splay progradation and floodbasin filling. The Cadotte splay complex, formed from the avulsively annexed Cadotte Channel, represents a small but typical area of the avulsion belt. Facies characteristics and interrelationships examined from auger borings and vibracores are presented to illustrate patterns of deposition and floodplain development. Avulsive flooding initially raised local water-surface elevations, deepening small existing lakes and creating new shallow (1-2 m deep) lakes on the floodplain surface. Deposition began when levees of the Cadotte Channel were breached, allowing water and coarser sediment to spread as sheet-like flows while finer sediment was carried and deposited farther into the basin. Later, as the proximal splay sheets coalesced and grew basinward, flows became channelized, and progradation of mouth bars into marshes and shallow lakes, together with deposition of interchannel fines, became dominant, forming coarsening-upward sequences. Further floodplain aggradation developed by overbank flooding when splay channels became stabilized and bordered by levees. Gradual abandonment was accompanied by de-position of organic-rich fine-grained sediment on top of the progradational sequences. Throughout the avulsion, the North Angling Channel has stood as an older alluvial ridge separating two broad areas of avulsive deposition. Its recent levee deposits now grade basinward into these new floodplain sediments composed of thicker heterogeneous, predominantly upward-coarsening, and rapidly deposited facies of the avulsion.

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