Several major avulsions of the Saskatchewan River have occurred in the Cumberland Marshes (east-central Saskatchewan) during the past few thousand years. The most recent avulsion occurred in about the 1870s, converting over 500 km2 of floodplain into a belt of anastomosing channels, splay complexes, and small lakes, a region that is still evolving today. The avulsion began near the tip of a large meander bend by following a small outflowing creek (Sturgeon River) which in turn followed an abandoned former channel of the Saskatchewan River. Flow began to permanently divert out of the Saskatchewan when a narrow strip of floodplain separating the Sturgeon River from the nearby Torch River became breached. Diversion into the connected Sturgeon-Torch began to increase sometime in the 1870s and probably culminated around 1882. The triggering event for the avulsion may have been a chute cutoff of the meander bend, shown by numerical modeling experiments to have significantly raised water-surface elevations at the avulsion site. Increasing flow diversion soon overwhelmed the smaller Sturgeon-Torch channel (now known as the New Channel), and several crevasse splays formed to help accommodate avulsive discharge. Sixteen kilometres downstream, most of the avulsive flow spilled out of the New Channel to form a shallow (~1 m), marshy floodplain lake which flowed eastward down the regional floodplain gradient to the basin presently occupied by Cumberland Lake. Since its inception, the avulsion-generated lake has become gradually infilled by prograding splay complexes fed by networks of anastomosing channels to characterize most of the present-day avulsion belt.