estuary - a water passage where the tide meets a river current.
(i think this is one of the most beautiful words we have in English..where fresh and salt water meet but still implies the individual identity of each)

The enameled face of the sea that changed imperceptibly in color, assumed towards the head of the bay, where an estuary opened, the blue whiteness of milk, in which little black boats which did not move entangled like flies.