Geomorphological evolution and ecological response of the River Ehen to the reconnection of a headwater tributary

This PhD thesis, funded by the Environment Agency and United Utilities, reports on the post project evaluation of a restoration initiative. The River Ehen (NW England) is home to the largest remaining population of the endangered freshwater pearl mussel in England. Concerns about the survival of the population has led the agencies to implement a catchment-scale rehabilitation project, of which the reconnection of a diverted tributary (Ben Gill) represents the centre-piece. The aim of this process-based restoration was to restore natural fluvial dynamics to improve habitat. The reconnection involved the creation of a new section of channel from the diversion point to the confluence. Monitoring the evolution of this section was crucial, to assess the volumes and frequency of sediment delivery to the Ehen. This was done using Structure-from-Motion photogrammetry from drone-based aerial imagery. Comparison of successive topographic models allowed computation of geomorphic changes, for the channel and the confluence bar. Ben Gill delivers both fine and coarse material which influence physical habitat quality. Comparison of suspended sediment loads pre- and post-reconnection revealed that these have increased by 65%, despite the relatively small size of the reconnected Ben Gill catchment (+1.2% in catchment area). This highlights the role of ephemeral headwater tributaries, also in temperate climate regions. The impact of this increased sediment load on the in-channel fine sediment storage is significant and depends on the relationship between flows in the main stem and those in the tributary. Only a fraction of the coarse material flux from Ben Gill is readily available for transport in the Ehen. Early signs of geomorphic adjustments are in line with pearl mussel habitat requirements although overshadowed by the increase in fine sediment deposition. Crucial to the appraisal of the success of this restoration is the continued monitoring of habitat conditions and of the pearl mussel population.