Depositional patterns following the 1870s avulsion of the Saskatchewan River (Cumberland Marshes, Saskatchewan, Canada)

The avulsion of the Saskatchewan River (Cumberland Marshes, east-central Sa skatchewan, 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 "over bank" sediments in alluvial floodplains presumed to be supplied by intermit tent flooding of dominant master channels. The North Angling and Cadotte ch annels are older pre-existing channels that were intercepted and appropriat ed by the expanding avulsion, Following annexation of the North Angling Cha nnel, stable cohesive banks inhibited channel enlargement and crevassing, f orcing 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 pr oximal deposition rates brought on by the larger flows. In contrast, the Ca dotte 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 Chann el, represents a small but typical area of the avulsion belt. Facies charac teristics and interrelationships examined from auger borings and vibracores are presented to illustrate patterns of deposition and floodplain developm ent. Avulsive flooding initially raised local water-surface elevations, dee pening small existing lakes and creating new shadow (1-2 m deep) lakes on t he 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 b ecame channelized, and progradation of mouth bars into marshes and shallow lakes, together with deposition of interchannel fines, became dominant, for ming coarsening-upward sequences. Further floodplain aggradation developed by overbank flooding when splay channels became stabilized and bordered by levees. Gradual abandonment was accompanied by deposition of organic-rich f ine-grained sediment on top of the progradational sequences. Throughout the avulsion, the North Angling Channel has stood as an older al luvial ridge separating two broad areas of avulsive deposition. Its recent levee deposits now grade basinward into these new floodplain sediments comp osed of thicker heterogeneous, predominantly upward-coarsening, and rapidly deposited facies of the avulsion.