The role of oxbow lakes in the off-channel storage of bed material along the Ain River, France

Incidents of chute cutoff redistribute floodplain sediment into rivers, causing downstream bar growth while simultaneously creating accommodation space for the storage of sediment within the floodplain in the form of oxbow lakes. Oxbows may be able to sequester enough sediment to balance the amount produced by chute incision, but the long-term consequences of chute cutoff on reach-scale sediment budgets have so far remained unclear. This has been due to a relative paucity of field observations that quantify the exchange of coarse sediment between the channel and floodplain. Here, we take advantage of a unique opportunity to document the sediment budget of a reach of the Ain River, France, that has experienced three recent incidents of chute cutoff. Monitoring of the river prior to chute incision allowed us to precisely quantify the rates of bed-material transfer over a thirteen-year period using a combination of bathymetric surveys, LiDAR data, and aerial photographs. The abandoned channels under study sequestered between 17 and 40% of the sediment introduced to the channel, with most of the rest of the sediment being stored within the river itself. Aggradation of the abandoned channels was not evenly distributed, instead occurring by the growth of point bars and thus implying that the abandoned channel planform may be an important control on aggradation rates. Our results make clear that although oxbows may provide a significant sink for bed material, the amount of sediment sequestered within them cannot compensate for the loading caused by chute incision. ► Chute incision introduces significant volumes of sediment to the channel. ► Off-channel storage of bed material within oxbows may equal 17–40% of the loading. ► The long term effect of chute cutoff is a net increase in bed-material loading. ► The deposits of chute-cutoff generated oxbows may consist primarily of bed material. ► Coarse aggradation occurs along point bars, suggesting the role of oxbow curvature.