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How do tides mix fresh water and salt water in estuaries?
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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 water and salt water is driven by the interaction of river flow and the ebb and flow of tides. Because fresh water is less dense than salt water, it typically forms a surface layer that floats above the denser seawater.1
The extent of mixing depends on the specific type of estuary:
- Partially mixed estuaries: The vigorous rise and fall of the tide generates strong turbulence, which causes partial mixing between the fresh water at the surface and the salt water below. This process often leads the river flow to entrain significant volumes of salt water, triggering a compensatory landward flow of seawater along the bottom.2
- Salt wedge estuaries: These exhibit minimal mixing. A wedge of salt water extends along the bottom, tapering toward the land. While mixing is limited, the boundary between the fresh and salt water layers allows the surface flow to entrain some salt water, which is compensated for by a slow, landward movement of salt water at depth.3
Additional mixing can also occur due to strong winds and internal waves propagating along the interface between the two water layers.1