Estuary

An estuary is the tidal mouth of a river where fresh water meets seawater, creating brackish water with a salinity gradient that varies with tides, river discharge, and weather. Estuaries are dynamic, productive ecosystems shaped by the interaction of fluvial, tidal, and atmospheric processes. They commonly exhibit vertical or horizontal salinity gradients, with fresher water overlying saltier water, though mixing can produce a range of patterns from well-mixed to salt-wedge conditions. The geometry of an estuary—such as drowned river valleys, fjords, or tectonically formed basins—together with tides and sediment supply, determines its hydrodynamics and sedimentation.

Estuaries support high biological productivity and diverse habitats, including mudflats, salt marshes, mangroves, and seagrasses. They serve as nurseries for many fish and invertebrates, provide feeding grounds for birds, and act as nutrient sinks and carbon reservoirs. They also offer services such as water filtration, shoreline protection, and opportunities for human activities like shipping, fishing, and recreation.

Human pressures include pollution and nutrient loading leading to eutrophication, habitat loss from development and dredging, dam construction reducing freshwater input and sediment supply, and climate-related sea-level rise. Conservation and management strategies focus on maintaining natural hydrology, restoring habitat mosaics, reducing pollution, and protecting watershed connections to preserve estuarine functions.