Estuarine areas, as the interface between rivers and the sea, are profoundly affected by upstream activities, and particularly by factors such as the over-abstraction of water.
Generally, estuarine habitat is being lost at rates that concern coastal scientists and mangers. Much of the decline of salt marsh and seagrass systems has been through some type of alteration to the flow of water to these habitats, such as dams, levees, dikes, dredge and fill operation, drainage, roadways, etc. It is important for resource managers to understand the importance of these habitats to the long-term support of fish populations.
The ecological functioning of estuarine ecosystems is critically dependent upon the complex and dynamic interplay between rivers and the sea, a factor which increases the vulnerability of such environments to changes both within catchments and in the sea. Activities affecting estuarine ecosystems include excessive water abstraction, resulting in a reduction of freshwater to estuaries; agricultural practices that lead to increased soil erosion and thus silt deposition in estuaries; urban or industrial development adjacent to estuaries; modifications to river and tidal flows through floodplain development and the construction of bridges, harbours, and dams; and pollution resulting from the diversity of activities occurring in catchments.
Estuaries are tidaly-influenced ecological systems where rivers meet the sea and fresh water mixes with salt water. They are crucial, highly productive transitional zones between land and water that provide: habitats, nurseries, water filtration, and flood control.