Salt-wedge estuaries are the most stratified, or least mixed, of all estuaries (Molles, 2002; Ross, 1995). They are also called highly stratified estuaries. Salt-wedge estuaries occur when a rapidly flowing river discharges into the ocean where tidal currents are weak. The force of the river pushing fresh water out to sea rather than tidal currents transporting seawater upstream determines the water circulation in these estuaries. As fresh water is less dense than saltwater, it floats above the seawater. A sharp boundary is created between the water masses, with fresh water floating on top and a wedge of saltwater on the bottom. Some mixing does occur at the boundary between the two water masses, but it is generally slight. The location of the wedge varies with the weather and tidal conditions. Examples of salt-wedge estuaries are the Columbia River in Washington and Oregon, the Hudson River in New York, and the Mississippi River in Louisiana.
In the animation below, the blue-colored fresh water flows from the river on the right-hand side of the image over a green-colored wedge of salty seawater as it moves out toward the ocean on the left-hand side of the image.