Loss of habitat and connectivity during drought

This study focuses on eco-hydraulics as a mean to assess the impact of drought on river ecosystems. The health of a river is influenced by many interacting natural and anthropogenic factors among which is flow (discharge). While all facets of the flow regime are important to river ecosystems, discharge has only an indirect effect. Indeed river organisms respond directly to river hydraulics in terms of: (i) available physical habitat (ie depth, velocity) created by the interaction between flow and channel morphology; (ii) river longitudinal connectivity (ie less opportunity to explore food sources, or to find most suitable conditions when connectivity is broken). During a drought, the decrease in discharge first cause a loss of physical habitat, then of connectivity. We chose to represent river hydraulics using Hydraulic Geometry (HG), which is a simple characterisation of river hydraulics as wetted width, mean water depth, and mean water velocity modelled as power functions of flow in natural rivers. We fitted HG models at about 3000 sites in England and Wales. Firstly, we aimed to understand better river hydraulic sensitivity to flow variability and, to assess how catchment properties can influence this. HG models coefficients were analysed against a wide range of catchment descriptors. Secondly, for a subset of sites covering a wide geographical area, we generated time series of depth, width, and velocity. We quantified typical habitat loss at sites during drought years. Site information was then aggregated at catchment level to assess how fragmented they are under drought conditions. Results were mapped. The approach was used with both historical and projected future discharge data.