| Summary: | The natural flow paradigm is based on the assumption that aquatic organisms have adapted to a range of flows. However, flow variability is currently not represented in water resource management in England and Wales. This thesis addresses the need to incorporate seasonal and inter annual flow variability in managing rivers for migratory salmonids. This study explores the degree of inter- and intra-annual flow variability at different spatial and temporal scales across the UK, using a novel approach to analysing natural variations in flow regimes. Principle components analysis and cluster analysis, were combined with the Indicators of Hydrologic Alteration approach to analyse 850 years of station flow data from 17 rivers within 3 regions. The analyses focussed on functional flows of know/suspected influence on salmonid populations. Atlantic salmon (Salmo salar) is in decline throughout most of its range, yet as a protected species legally requires measures to improve populations. This thesis assembled multi-decadal datasets in the form of rod catch data, as a proxy to represent salmon populations. However, analyses provided limited meaningful insight into the relationship with flow. It is suggested that the multiple pressures impacting salmon populations, within freshwater, intertidal and marine stages of the lifecycle, made it challenging to isolate the impact of flow. In light of the uncertainties and with the pressure on water resources set only to increase, it is essential that natural flow variability is understood and incorporated into river management. This will provide protection by increasing heterogeneity and possible refugia from extreme events making the Atlantic salmon populations more resilient and able to adapt to anthropogenic and environmental pressures going forward. This study questions if other management actions, such as juvenile habitat enhancement, could also be influential.
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