Timescales, mechanisms, and controls of incisional avulsions in floodplain wetlands: Insights from the Tshwane River, semiarid South Africa

Abstract

Avulsion (relocation of a river course to a new position) typically is assumed to occur more frequently in rivers with faster sedimentation rates, yet supporting field data are limited and the influence of sedimentation rate on avulsion style remains unclear. Using analysis of historical aerial photographs, optically stimulated luminescence dating of fluvial sediments, and field observations, we document three avulsions that have occurred in the last 650years along the lower reaches of the semiarid Tshwane River in northern South Africa. Study of the modern river and abandoned reaches reveals that a downstream decrease in discharge and stream power leads to reduced channel size and declining sediment transport capacity. Bank erosion drives an increase in channel sinuosity, leading to a decline in local channel slope, and to a further decrease in discharge and sediment transport. Local sedimentation rates >10mma−1 occur within and adjacent to the channel, so over time levees and an alluvial ridge develop. The resulting increase in cross-floodplain gradient primes a reach for avulsion by promoting erosion of a new channel on the floodplain, which enlarges and extends by knickpoint retreat during periods of overbank flow. Ultimately, the new channel diverts the discharge and bedload sediment from the older, topographically higher channel, which is then abandoned. Our findings support the assumption that avulsion frequency and sedimentation rate are positively correlated, and we demonstrate that incisional avulsions can occur in settings with relatively rapid net vertical aggradation. The late Holocene avulsions on the semiarid Tshwane River have been driven by intrinsic (autogenic) processes during meander belt development, but comparison with the avulsion chronology along a river in subhumid South Africa highlights the need for additional investigations into the influence of hydroclimatic setting on the propensity for avulsion.