They are tides, winds, waves, and river runoff. The tides represent the most important force, often causing large-scale turbulence and mixing in the estuary. Winds are considered to be of lesser importance. They can be a major force, however, where tidal range is small. Winds also have greater effects on shallow estuaries, especially those with large open areas.
Winds also create various surface and internal waves, which can contribute to estuarine mixing.
River water is more buoyant than ocean water. As it flows seaward in the surface layer, and the denser salt water flows landward in the bottom layer, some mixing can occur. For example, the monthly cycle of spring and neap tides can greatly alter the character of an estuary where tides are the major mixing force. During spring tides, when tidal ranges are greatest, increased currents and tidal sloshing can speed up the mixing process. During neap tides, on the other hand, tidal ranges are smaller and might have little effect on mixing, resulting in stratification of the estuary.