Bay of Belfalas
From Tolkien Gateway
The Bay of Belfalas was a great gulf of the Great Sea lying to the south of Gondor, within whose waters stood the island of Tolfalas; the River Anduin flowed into the Bay. Directly north of it was the Gondorian region of Belfalas.[1]
During the Drowning of Númenor, the Bay of Belfalas was much filled at the east and south, and land upheaved around it. Anduin carved a new path by many mouths to the Bay.[2]
Tolfalas was almost destroyed, and was left at last like a barren and lonely mountain in the water not far from the issue of the River.