Flow History Effects on River Bifurcation Dynamics in a Himalayan River

Abstract Dynamic gravel bed rivers experience frequent changes in channel position and flow distribution between branches, which can alter the location and extent of flooding. Changes in flow routing can significantly impact livelihoods, habitats and infrastructure in adjacent floodplains. Here, we test whether variations in seasonal discharge patterns cause instability in channel position and flow distribution in a gravel bed river system around a major channel bifurcation. Satellite images of the Karnali River, Nepal, were assessed over a 30‐year period to identify changes in channel position and flow partitioning downstream of the bifurcation. These observations were compared with daily discharge records to establish whether the sequencing of peak monsoonal flows coincided with geomorphic changes in the river. Changes to flow partitioning trends were consistently preceded by monsoon seasons with two large peak flows, suggesting a history‐dependent threshold in the channels. To explain this observation, we use grain‐size data from gravel bars that reveal variable grain clustering and bed armoring across the channel network. We propose that two high discharges are needed to transition between phases of bifurcation stability or instability, where the first event acts to break down the bed armor layer, allowing the second high flow to drive enlargement/closure of branches and reworking of the bed. Our findings suggest that flow sequencing is an important driver in flow distribution and stability at bifurcations in gravel bed rivers. Although the focus is on Himalayan rivers, the findings may be of relevance in other areas that experience changing seasonal flood regimes.