For decades, people understood the rudiments of how idealized estuaries work: fresh water flows outward above saltier water that intrudes into the estuary. At the same time, mixing smooths out contrasts, so that the ultimate shallow outflow water is much saltier that the upstream riverine inflow had been. But, this vision is qualitative, so that it is hard to predict or diagnose flow structures or the outflow salinity. Hansen and Rattray provided a major step forward in an age before any sophisticated numerical models could deal simultaneously with the many competing effects...
Brink, Kenneth H., and Maurice Rattray. 2021. "Editor's Commentary: Gravitational circulation in straits and estuaries." Journal of Marine Research 79, (2). https://elischolar.library.yale.edu/journal_of_marine_research/502