Designing flow releases from reservoirs for improving ecological conditions in regulated rivers

Dams and reservoirs significantly modify and damage river ecosystems by disrupting the natural flow, sediment, and thermal regime. In these heavily modified rivers, releasing water from reservoirs can control the hydraulic habitat conditions for aquatic species, sediment movement, and river temperature of the river downstream and can sustain the ecological integrity of the river. Nevertheless, the effects of released flow characteristics, including flow magnitude, duration, rate of change, and timing on spatiotemporal habitat conditions and their importance in influencing complexity flow patterns, sediment movement, and river temperature remain relatively unquantified. This thesis examines flow releases from reservoirs to improve the ecological condition of heavily modified rivers for brown trout. It is composed of three main results chapters in which numerical and mathematical logic modelling approaches were applied: (i) investigating the spatiotemporal impact of peak discharge and rate of change of the experimental flood hydrograph on flow and sediment dynamics; (ii) assessing hydraulic and thermal habitat conditions influenced by dam releases; and (iii) quantifying the ecological impacts of sudden dam release variations. Overall, this thesis highlights that considering factors such as sediment transport, water temperature, channel shape, and up-/down-ramping rates of flow releases combined with operational and resource availability constraints is essential in limiting impacts on instream biota, including fish. The findings of this thesis can be used to inform the e-flow design process and the development of strategies to better manage the hydrologic regime and improve the ecological functioning of the river system. Incorporating such processes into e-flow studies can help design sophisticated e-flows design to sustain ecological integrity in heavily modified rivers.