Autocyclic (avulsion‐induced) base‐level change, its upstream and downstream controls, and effects on floodbasin sedimentation: The 1870s avulsion of the Saskatchewan River, Cumberland Marshes, Canada

Abstract

This study examines centennial‐scale hydrological and sedimentological effects of floodplain inundation by avulsion and its upstream and downstream controls. The 1870s avulsion in Cumberland Marshes diverted the Saskatchewan River flow towards Cumberland Lake, a local base level. It invaded a poorly drained sub‐basin of Cumberland Marshes floodplain linked to the parent Saskatchewan River by two small outlets in the resistant substrate. The rapid increase in inflow (~5× on average) during the earlier stages of the avulsion resulted in the base‐level rise and floodplain inundation by the avulsion lake. Since the early 20th century, the forced regression of the avulsion lake occurred, caused by ~5× outlet channel enlargement by ‘hungry‐water’ outflows, whereas the mean lake inflows experienced little change. The avulsion lake served as an effective sediment trap and was filled by predominantly progradational sandy and silty avulsion deposits up to 3–4 m thick, covering about 700 km2. Elsewhere, fluviodeltaic settings with ‘negative relief’ and limited hydrologic connectivity with the rest of the floodplain may be prone to avulsion lakes that form if the rates of inflow increased by avulsion exceed the rates of outflow. Avulsion lakes can last for ~100 years or more before they drain and/or become filled by overbank sediments. On well‐drained floodplains, inundations by avulsions are expected to be short‐term and result in little progradational deposition. This study demonstrates that in some local hydrographic basins, base level becomes a variable of an evolving avulsion rather than its fixed external control. Although avulsion‐induced base‐level changes are short‐lived, they affect 102–103 km2 of a floodplain and occur rapidly, accompanied by high aggradation rates.