Changes in physical habitat in an ecologically important upland river following tributary reconnection. (#72)
Despite their accepted importance, very few river restoration projects incorporate before-after monitoring programmes. Here we report on monitoring designed to assess the effects of the reconnection of a formerly diverted tributary on fluvial processes and instream habitat conditions in an upland river (River Ehen, NW England). The reconnection was conceived to help improve physical habitat conditions for important population of the endangered freshwater pearl mussel Margaritifera margaritifera.
Fluvial processes in the Ehen prior to and following the reconnection were monitored using a variety of sampling, tracer-based and remote monitoring methods. Changes in suspended sediment dynamics are described in this paper, along with changes in the frequency of movement and travel distances of coarser bed material. Changes in the amount and composition of fine sediment stored on the bed surface of the Ehen are also outlined, along with variation in subsurface dissolved oxygen levels. To understand its evolution, sedimentological changes and related geomorphic adjustments in the newly created tributary channel were also monitored, using an Unmanned Aerial Vehicle in concert with tracer studies.
Changes sediment dynamics were evident in the Ehen within the first year of the reconnection. New morphological features developed rapidly (notably a confluence bar) as a result of the large volume of coarse material arriving from the tributary. Successive DEMs indicated that the tributary channel underwent marked changes, associated with successive high flow events. The changes in the Ehen, especially the provision of relatively coarse, loose sediment, are predicted to improve instream conditions for mussels.