Environmental gradient relative to oxbow lake-meandering river connectivity in Zoige Basin of the Tibetan Plateau

Abstract

Oxbow lakes generated from meandering river cutoffs are an indispensable component of riparian ecosystem. This study aimed to examine whether predictable patterns in water properties arise relative to oxbow lake geometry and lake-channel connectivity. We selected 24 oxbow lakes along a meandering river belt in Zoige Basin, the Tibetan Plateau, to conduct field work at the beginning (May) and the end (September) of the flood season, 2019. Six parameters (pH, dissolved oxygen (DO), conductivity, total dissolved organic carbon, nitrogen and phosphorus) were measured to represent water property. Lake surface area, length, width, length/width ratio, distance to active channel, and curvature were measured to represent lake geometry. Shrinkage ratio, defined as current lake length/initial lake length, was measured to represent lake-channel connectivity between oxbow lakes and active channel. We found an environmental gradient relative to connectivity. As oxbow lakes aged and gradually disconnected from the channel, increased DO and nutrients drove the lakes towards eutrophication. This trend however was disturbed by (i) flood pulses that diluted nutrient concentrations in oxbow lakes and narrowed the differences among different types of lakes and the channel through altering lake-channel connectivity, and (ii) lateral meander migration passed through older lakes such that regulated water properties by reconnecting older lakes to newly formed lakes. Maintenance or restoration of connectivity thus should be of great importance to prevent oxbow lakes from eutrophication in meandering river belts in Zoige Basin.