Meander chute cutoff at an alluvial river facilitated by gypsum sinkholes

Collapse sinkholes are common in areas where gypsum strata underlie rock that is incompetent to bridge the cavities created by the groundwater dissolution of gypsum. On floodplains these can be obscured due to fill by overbank deposits, and channels tend to migrate towards larger enclosed depressions. This study utilises GIS to analyse the influence exerted by small collapse sinkholes on the channel dynamics of the alluvial River Ure, UK. At Ripon Loop, a large compound meander bend, sinkhole area and channel dynamics have been tracked over >160 years from multiple datasets, including historic maps, aerial imagery, lidar and a Structure-from-Motion photogrammetry dataset acquired for this study. This indicated two distinct populations, firstly long-lived, constant-sized sinkholes and secondly dynamically growing holes aligned with the contact of bedrock and unconsolidated materials of a buried valley at depth where artesian water saturated with sulphate enters the latter. Frequent overbank flows of the River Ure across the neck of the bend have interacted with sinkholes, e.g. by deposition of bedload in the down-valley end of the holes and localised headward incision on the opposite side due to the relatively high gradient between holes and the downstream limb. The location of holes has conditioned the incision of small shallow channels over at least 7 years, before cutoff occurred over several floods in 2019. Initially overbank flow traversed the neck via the chain of sinkholes before widening and deepening of the channel captured all the surface flow of the River Ure, resulting in the mobilisation of more than 20,000 m3 of sediment. Allogenic influences on fluvial systems in relation to meander dynamics are often neglected and this study is the first to link floodplain sinkholes with detailed mechanisms of a chute cutoff, hereby indicating a potential feedback to sinkhole dissolution processes