Establishing the Hydrological Controls on Water Surface Area Variations in Oxbow Lakes

Abstract

Oxbow lakes are iconic fluvial landforms found in the floodplains of meandering rivers around the world. Their formation is associated with meander cutoff, a process that excises sections of river channel to optimise the downstream transmission of water and sediment. Overbank floods and conveyance through tie channels maintain some hydrological connectivity, but lakes are generally considered to passively infill until they are terrestrialised. Here, a suite of 64 lakes across two meandering rivers in the Bolivian Amazon Basin are used to demonstrate the hydrological dynamism of oxbow lakes by quantifying interannual variations in lake water surface area (WSA), using the modified Normalised Difference Water Index (mNDWI) on an archive of Landsat images, and evaluating the mechanisms controlling these changes using remotely sensed rainfall data and geospatial analysis. The majority of lakes (75%) decreased in size over the study period, while 25% increased in size. The results suggest that WSA variations are controlled by proximity to the active channel, with the magnitude of these variations being set by mechanisms of connectivity. Lakes connected by tie channels experienced WSA changes up to 3.9 times larger than lakes with no visible connection mechanisms. Incursion lakes displayed similar WSA changes to those with tie channels, while isolated lakes were found furthest from the mainstem and had the smallest range of WSAs. Chute lakes experienced a wider range of WSA change (−95% to +281%) and were more strongly controlled by mainstem proximity than neck lakes. Connectivity between the river and oxbow lakes is essential for governing lake hydrodynamics, and tie channels provide the critical conduit by which water can be transmitted deep into the floodplain.