Estuary
This is where a freshwater river meets the ocean. The main feature of an estuary is that the water becomes “brackish,” which is a salinity halfway between freshwater and the ocean – so, “half salty,” basically.
An estuary can be a simply river delta, or a large ecosystem of marshes and barrier islands.
From the Latin words for “tide” and “boil,” refering to the physical action on the moving river water mixing with the still ocean water.
Several resources commented on how productive estuaries were for wildlife, but the only reasoning I could find was from Brittanica:
On average, estuaries are biologically more productive than either the adjacent river or the sea, because they have a special kind of water circulation that traps plant nutrients and stimulates primary production.
Why I Looked It Up
I went to a conference in Washington DC. Whenever I would walk to the convention center, I walked past a sign at a hotel entrance that said “Estuary.” I realized that I didn’t really know what the word meant.
When I looked it up (to write this), I got to wondering about the usage at that hotel in DC. I thought, “Maybe it’s a generalized usage, because the road meets the hotel parking lot? Like, maybe this is a neologism for this kind of thing?”
Nope. Turns out the hotel restaurant was just named “Estuary.”