Does Saltwater Sit on the Bottom of a River?

Contrary to what one might assume, saltwater does not simply pool at the bottom of a river. In estuaries where freshwater rivers meet the ocean, the denser saltwater pushes inland beneath the lighter freshwater, forming a distinct wedge that hugs the riverbed. This phenomenon, known as a salt wedge, can extend many miles upstream. However, the exact behavior varies significantly depending on the river's flow rate, the volume of freshwater discharge, and the shape of the riverbed. Strong river flows can push the salt wedge back toward the sea, while drought conditions allow it to creep further inland. Understanding this dynamic is crucial for managing freshwater supplies, ecosystems, and infrastructure in coastal regions.