Relict channel topography increases mobility on meandering rivers

The propensity of meandering rivers to change flow path is a well-observed phenomenon. Meander bends migrate across their floodplains in response bank erosion and point bar deposition, progressively increasing channel sinuosity and decreasing the downstream flow gradient. This process ultimately leads to meander cutoff – where the river forms a localised avulsion to improve the downstream flow and sediment conveyance. On freely meandering rivers, these relict channels – or oxbow lakes – provide a mechanism through which rivers can bypass the excavation of floodplain material, to form a new path, and recycle formerly active sections of river channel. Here, I present observations of channel migration and cutoff formation on two meandering rivers in the Bolivian Amazon Basin and quantify how these fundamental processes evolve in response to the surrounding floodplain relief. I use a semi-automated approach to quantify channel migration and cutoff formation over 3.5 km long river valley segments for a 35-year remote sensing record and couple this to the existing – and evolving – relict channel population in the floodplain. Several hundred relict channels are identified in the floodplains of each river, with cutoff hotspots correlating to areas with the greatest lake presence. Over 100 individual cases of incursion, where the river reoccupies relict channel topography, were identified across the two rivers suggesting that this is a common process along actively evolving river corridors and has implications for the rates and calibre of floodplain material cycling and the remobilisation of particulates (e.g., contaminants) stored in the floodplain.