Chronology and controls of avulsion along a mixed bedrock-alluvial river

Avulsions are most characteristic of aggrading fluvial systems, but along the Klip River, subhumid eastern South Africa, abandonment and re-establishment of meander belts occurs under conditions of negligible vertical aggradation. Channel banks are fully alluvial, but the bed is grounded on sandstone and shale, so that during meander migration, alluvium 2–4 m thick is deposited by lateral accretion on a near-planar bedrock surface. Abandoned meander belts up to 4 km long are present where floodplain gradient increases by a factor of 2–3 and floodplain sediments change from dominantly mud to sand. Optically stimulated luminescence (OSL) and radiocarbon ages from abandoned channel fills indicate that avulsions occurred at ca. 30 ka, ca. 15 ka, ca. 11 ka, ca. 4.5 ka, and ca. 1 ka. The lack of close correspondence between these ages and regional paleoenvironmental changes suggests that avulsions have not been allogenically forced but rather have occurred autogenically during meander-belt development. The absence of crevasse splay complexes indicates that avulsions are primarily incisional phenomena whereby rainwater and overbank floodwater that originates on the upstream muddy floodplain drains back to the channel through low points in levees or bank tops, initiating new channels that erode headward through the floodplain to connect with the original channel farther upstream. OSL ages and field evidence indicate that establishment of meandering along a new channel takes several millennia. European settlement in the valley in the late 1800s initiated an ongoing avulsion; channel abandonment over a 2–3 km reach is proceeding alongside rapid headward incision of a new channel, resulting in radical flow and sediment redistribution. The findings support previous suggestions that aggradation rate is a primary control on avulsion frequency and style.