Linked from
The 88 pages that link to Estuary, each with the reason it gives.
RiverBroader topic: An estuary is a common receiving environment for river water and sediment.
Brackish waterBroader topic: Estuaries are a common setting for brackish water.
River mouthBroader topic: Many river mouths broaden into tidal zones where fresh and salt water mix.
Saline waterBroader topic: Its shifting freshwater–seawater mixture creates a salinity gradient.