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How do rivers and seawater mix in an estuary?
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estuary
Four main types of estuaries: (A) salt wedge estuary, (B) partially mixed estuary, (C) vertically homogeneous estuary, and (D) fjord (black arrows indicate salt water and white arrows fresh).
In an estuary, the mixing of fresh river water and salt water is primarily determined by the balance between river discharge and tidal energy. Oceanographers classify these systems based on the extent of this mixing:1
- Salt wedge estuaries: Dominated by river flow, these systems feature minimal mixing. Fresh water flows over a distinct, wedge-shaped layer of denser salt water that tapers toward the land. The interface between the two is well-defined, with little mass transfer.21
- Partially mixed estuaries: Increased tidal energy generates strong turbulence, causing the two water types to mix. The river flow entrains significant volumes of salt water, creating a compensatory landward flow of seawater along the bottom.31
- Vertically homogeneous estuaries: Where tidal flow is strong and river flow is weak, total mixing occurs. This eliminates the boundary between fresh and salt water, resulting in nearly constant salinity from the surface to the bottom at any given location.14
Generally, higher river discharge leads to weaker mixing and a more pronounced salinity gradient, while stronger tidal currents promote more thorough blending of the waters.1