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Temporarily closed estuaries require seasonal opening to tidal flows to maintain normalecological processes. Each estuary has specific environmental flow (EFlow) requirements basedon the relationship between freshwater inflow, coastal dynamics, rate of sandbar formation,and the open/closed state of the mouth. Key abiotic processes and ecosystem services linkedto mouth state were highlighted. We reviewed completed EFlow requirement studies for temporarilyclosed estuaries in South Africa and found that the formulation of these requirements shouldconsider the timing and magnitude of flows in relation to the morphology of an estuary, its mouthstructure, catchment size, and climate. We identified ten key principles that could be adapted tosimilar systems in equivalent climatic settings. Principle 1 recognizes that each estuary is unique interms of its EFlow requirements because size, scale, and sensitivity of core elements to freshwaterinflow are specific for each system; EFlows cannot be extrapolated from one estuary to another.Principle 2 highlights the importance of baseflows in keeping an estuary mouth open because a smallreduction in flow can cause the mouth to close and alter essential ecological processes. Principle 3outlines the role of floods in resetting natural processes by flushing out large volumes of sedimentand establishing the equilibrium between erosion and sedimentation. Principle 4 emphasizes the needfor open mouth conditions to allow regular tidal flushing that maintains water quality throughreducing retention times and preventing the onset of eutrophic conditions. Principle 5 advisesartificial breaching to be practiced with caution because execution at low water levels encouragessedimentation that reduces the scouring eect of flushing. Principle 6 holds that elevated inflowvolumes from wastewater treatment works or agricultural return flows can increase the frequency ofmouth opening and cause ecological instability. Principle 7 states that water released from dams tosupply the environmental flow cannot mimic the natural flow regime. Principle 8 specifies the needfor short- and long-term data to increase the confidence levels of EFlow assessments, with data tobe collected during the open and closed mouth states. Principle 9 advocates the implementation ofa monitoring program to track the achievement of EFlow objectives as part of a strategic adaptivemanagement cycle. Finally, Principle 10 recommends the adoption of a holistic catchment-to-coastmanagement approach underpinned by collaboration with regulatory authorities and stakeholdersacross a range of sectors. These principles can be used to guide the formulation and managementof EFlows, an essential strategy that links the maintenance of estuarine ecological integrity withsocial well-being.

Ten Principles to Determine Environmental Flow Requirements for Temporarily Closed Estuaries