Tidal channels commonly cut into lagoonal deposits as the location of concentrated flow migrates back and forth. Lagoons have extremely variable water chemistry depending on climate. If the climate is dry and a river does not flow into it, they tend to be evaporitic and hypersaline. The types of organisms living in the lagoon is a good indicator of local climate. As sea level rises relative to the land, they shift one way, and vice versa.
Estuaries are a special end member of lagoons.
They are river mouths that have been drowned by relative sea level rise and contain a mix of fresh and salt water. Both marine and fluvial processes influence them. With long term global sea level rise, many river mouths were inundated with sea water and became estuaries which are very important ecological sites. We had more than geologically typical until we started filling them in. Tidal range is very important for estuaries.