Estuaries are defined as bays where freshwater mixes with seawater. Estuaries form where rivers widen at the coast. Because freshwater is less dense than saltwater, the interaction between the two usually involves some stratification , whereby the fresher water overlies salt water. This tendency can be counteracted by turbulence due to tidal currents, which can be strong enough to break down stratification. Stratification is strongest in deep estuaries with weak tidal flow.
River flow enters the fjord and flows outward at the surface, entraining some salt water as it goes.
This means that surface salinity increases going seaward, and also that flow near the bottom must be into the estuary. A salt wedge estuary is a more typical river-built estuary in which tidal stirring is weak and river flow high. In this case the river flow holds the salt water back and tends to flow out over the top of it, again entraining salt water from below. Because of the density gradient from the ocean to the estuary, seawater tends to flow in under the freshwater. This two-layered gravitational circulation is important in salt transport and also in maintaining sediment particles in the estuary to create an estuarine turbidity maximum .