Hydrologic connectivity and morphologic variation of oxbow lakes in a pristine alpine fluvial system

Abstract

The lifespan of oxbow lakes in a meandering system typically exhibits considerable disparity due to the diverse evolutionary processes that lead to their terrestrialization. Although oxbow lakes generally undergo a long-term gradual shrinking trend, their hydrologic connectivity and morphologic variations under variable hydrologic conditions of the river are not well understood. Here we focus on a group of oxbow lakes in a pristine alpine fluvial system in the Qinghai-Tibet Plateau to examine the hydrogeomorphic response of oxbow lakes to a variable hydrologic regime of an active meandering channel. We find that the oxbow lakes are characterized by extensively developed compound forms and connecting “tie” channels and that the lateral migration of the main channel discernably impacts lake morphology and evolution. With increasing discharges in the main channel, the oxbow lakes generally show increases in length under relatively lower flows (RI < 10 years) and more dramatic increases in width under higher flows (RI > 10 years), which collectively lead to their expansion regardless of their hydrologic connectivity. This implies hydrogeomorphic processes other than surface water connection between oxbow lakes and the main channel in adjusting water levels of the lakes. To quantify hydrologic connectivity, we propose a probability-based index and find the overall low hydrologic connectivity of the studied oxbow lakes, owing to the unique paleogeographic setting of the study area. The infrequent but pulsed sedimentation within the lakes and the relatively slow lateral migration of the meandering channel catalyze an inactive floodplain of the study reach.