UNDERSTANDING AND ADDRESSING THE ECOLOGICAL IMPACTS OF FLOW IMPOUNDMENT FOR RIVER SYSTEMS IN NORTHERN ENGLAND

Many rivers have undergone flow modification by impoundments to provide services such as water supply and hydropower. There is an established consensus that typical modified flow regimes do not sufficiently cater to the needs of downstream ecosystems, due to species having adapted to natural flow conditions. This may lead to changes in the biodiversity and functional composition of ecosystems, potentially compromising water quality and other river system services. More must be done to understand the relationship between flow and in-stream ecology, in order to mitigate the impacts of flow modification. The development of efficient methods of ecology-flow assessment is vital in order to meet current and future legislation, whilst considering other stakeholders and maintaining the resilience of the local water supply. This thesis combines statistical approaches applied to public datasets, and combined ecological-hydraulic modelling at a case study site, to propose environmental flow regimes. Univariate and multivariate analyses were performed on flow and macroinvertebrate sampling data from sites across northern England. The mean annual frequency of high flow events was identified as a particularly influential driver of functional composition and biodiversity metrics. Field data was gathered and a hydraulic-ecological model was also developed for a selected case study site in order to predict the responses of selected indicator species to flow. Spatial and temporal distributions of habitat quality with respect to flow were generated, allowing the impacts of various flow inputs to be assessed. These findings were integrated in order to generate recommended flow regimes for the case study site. It was demonstrated that the proposed regimes met or improved upon ecological metrics relative to impoundment outflow data, whilst also conserving significant quantities of water. Outcomes from this research demonstrate the potential of habitat suitability models, supplemented by knowledge of ecological-flow relationships, to inform environmental flow design decisions.