Anatomy of an avulsion

Abstract

The Cumberland Marshes in east‐central Saskatchewan, Canada, occupy over 5000 km 2 and contain a variety of active and abandoned fluvial features, including straight to sinuous isolated channels, anastomosed channel systems, levees, and crevasse splays in addition to marshes, lakes and bogs. In 1873, an avulsion of the Saskatchewan River diverted most of its flow into a portion of the Cumberland Marshes (locally termed the breakout area), and altered the alluvial terrain as the invaded wetlands adjusted to the influx of sediment and water. These adjustments continue today, and so far over 500 km 2 of wetlands have been affected by the avulsion. Avulsion‐controlled modification of the wetlands involves the initiation and evolution of crevasse splays and splay complexes. Three intergradational forms are recognized, each associated with characteristic sand‐body geometries. Stage I splays are small, lobate in plan, crossed by unstable distributary channels, and form wedge‐shaped sheets which depositionally overlie fine‐grained, organic‐rich wetland sediments. Stage II splays and splay complexes evolve both spatially and temporally from Stage I splays. They are larger, contain dense networks of anastomosed channels, and form disconnected tabular sand bodies or continuous sand sheets, some of which incise underlying wetland sediments. Stage III splays develop from either Stage I or II splays and contain few but stable anastomosed channels that deposit isolated stringer sands encased in fine‐grained floodplain sediments. Although sand bodies deposited by splays comprise important components of the evolving floodplain, various fine‐grained facies occurring in levees, shallow lakes, abandoned splay channels, and interchannel floodplains dominate the avulsion deposits. The post‐1873 record of deposition and terrain modification in the breakout area suggests four stages of floodplain evolution following avulsion. In the initial avulsion stage,new channels and splay complexes increase in numbers rapidly as diverted discharge of water and sediment overwhelm the adjacent floodbasin. The anastomosed stage is characterized by an approximate balance between rates of new channel and splay development, and abandonment of old ones. This stage continues for as long as new floodplain areas are invaded. The rate of new splay development eventually decreases as accessible floodplain becomes aggraded, forcing a higher rate of channel abandonment and concentration of remaining flow into fewer but larger channels (reversion stage). The result of reversion is eventual return to a single channel stage,completing the avulsive sequence and initiating a new alluvial ridge. For the Saskatchewan River, this final‐stage single channel will likely produce a meander belt which occupies only a portion of the more extensive avulsion belt which preceded it.