Exposed riverine sediments (ERS) in England and Wales: Distribution, controls and management

The invertebrates of natural and semi-natural floodplains are diverse, often rare, and important functionally. Early successional patches formed by flood-deposited, coarse, exposed riverine sediments (ERS) are an important habitat in these riparian ecosystems. Extensive channel modifications and engineering works have removed ERS from much of the European river network. This vulnerability, coupled with their role in strengthening biodiversity and associated ecosystem function, makes it important to catalogue remaining ERS distributions and understand controlling factors. This paper aims to: (1) provide the first systematic, modern assessment of the extent and distribution of ERS in England and Wales; (2) characterize geographical and hydrological features of ERS-rich systems; and (3) model the physical and artificial factors determining the complexity and abundance of ERS in England and Wales. River flow regime analysis and generalized linear modelling were combined to identify variables influencing ERS habitat complexity and abundance. ERS predominates in the north and south west of England and Wales, where headwaters are at higher altitudes and substrates are predominantly glacial, alluvial or sedimentary. Local complexity (average number of individual ERS patches per km) was related to stream power and the potential for sediment transport as reflected by stream slope. ERS habitat area declined with the presence of headwater abstraction, but otherwise ERS was not associated with any identified hydrological regime. However, all rivers were strongly seasonal and characterized by sufficient flashiness to ensure high-flow events throughout the year. These results underline the strong natural controls on the distribution and complexity of ERS, but also that habitat provision can be suppressed by human activities, which has implications for conservation practice.