Flow regulation is widely recognized as affecting fluvial processes and river ecosystems. Most impact assessments have focused on large dams and major water transfer schemes, so relatively little is known about the impacts of smaller dams, weirs and water diversions. This paper assesses sediment dynamics in an upland river (the Ehen, NW England) whose flows are regulated by a small weir and tributary diversion. The river is important ecologically due to the presence of the endangered freshwater pearl mussel Margaritifera margaritifera, a species known to be sensitive to sedimentary conditions. Fine sediment yield for the 300-m long study reach was estimated to be 0.057 t km(-2) year(-1), a very low value relative to other upland UK rivers. Mean in-channel storage of fine sediment was also low, estimated at an average of around 40 g m(-2). Although the study period was characterized by frequent high flow events, little movement of coarser bed material was observed. Data therefore indicate an extremely stable fluvial system within the study reach. The implication of this stability for pearl mussels is discussed.

The freshwater pearl mussel Margaritifera margaritifera has declined across its range but current knowledge of its physical habitat requirements falls short of that needed to confidently guide habitat management and conservation programmes. This paper critically reviews knowledge of the physical habitat requirements of M. margaritifera and sets out a research agenda to address existing knowledge gaps. Present information on the hydraulic, sedimentary and oxygen requirements of M. margaritifera is based largely on descriptive or correlative field studies. Consequently, little is known of the processes that control the correlations observed between habitat conditions and pearl mussel abundance. Furthermore, a pressing concern is the need for standardization of field and laboratory techniques, notably in the context of sediment and hyporheic dissolved oxygen, where the variety of field and analytical techniques applied to date limits transferability. The review identifies nine important research questions and outlines approaches from environmental hydraulics, fluvial geomorphology and fisheries ecology that can be used to address them. Research specifically focused on the role of high and low discharges may help elucidate the hydrological, fluvial and bio‐energetic processes that influence M. margaritifera distribution and recruitment. Research is also needed to assess the role of mussels as engineers. The review outlines opportunities to apply new approaches to improve characterization of physical habitat conditions and assess how these conditions influence mussels. Improved characterization will provide a firm evidence base for habitat management and mussel conservation. Copyright © 2014 John Wiley & Sons, Ltd.

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