Along with the White Mountains and the Ephel Dúath, the Bay of Belfalas (S."Côf Belfalas) defined the natural boundaries of southern Gondor. From Ras Morthil on the tip of Andrast in the west to Hyarmendacil's Cape south of Umbar, the windy Bay of Belfalas shielded southern Gondor from the endless waves of the Belegaer beyond. Named after the rocky peninsula of the princes, the Bay was said by the Dúnedain to be a dwelling place of Uinen the Maia, guardian of Pelargir and the line of Imrazôr of Dol Amroth; whereas the violent waters of Belegaer were reckoned to be the province of her husband Osse. When the two quarreled, storms ravaged the otherwise relatively placid waters of Belfalas Bay.
- Type: Bay
- Area: 105,000 square miles.
- Depth: average 35 fathoms; deepest point 165 fathoms; shoals and reefs common along the coasts of Lebennin, Dor-en-Ernil, Anfalas, and Tolfalas.
- Waves and Wind: average wind speed: 2-7 knots; average wave height 1-4 feet.
- Climate: average annual rainfall 40-60 inches; mild annual temperature 50-60°F; average low: Narwain 35°F; average high Cerveth 85°F
- Composition: The entire Bay of Belfalas rested on Endor's continental shelf. The principal current was a warm counterclockwise flow known as the Southron Stream; it encountered the cold Great Stream (descending from the North) off the Cape of Andrast, creating turbulent seas there.
- Notes: The Bay of Belfalas was fairly shallow (averaging 35 fathoms, and reaching a maximum depth of 165) due to the fact that it rested upon the great continental shelf of Endor.
Warm currents moving up along the coasts of Harad encircled the bay in a counterclockwise direction, until they encounter the cold waters of the Great Stream, descending from Eriador in the north; it was this clash of contrary currents that generated the particularly unruly waters off the coast of Andrast, the bane of many a mariner.
Its waters were home to a wealth of marine life, providing sustenance for many of the people of Gondor and Harondor. Seals were a common denizen of the bay, as were dolphins, the friends of Uinen. Gulls and other sea-birds proliferated on the cliffs of the rocky coastline and pearl-bearing mollusks were plentiful in the shallow waters of Methed Lebennin. Occasionally, deep-sea dwellers would find their way into the bay. Whale migrations were an annual event along the western coasts of Anfalas, and scores of sea-turtles lay their eggs in the secluded coves of Tolfalas. There were some very dangerous predators present, including sharks, grey sperm whales, and giant squid. Normally, the seas were calm and the winds were fair on most of the Bay of Belfalas, but hurricanes were known to cause tremendous amounts of damage in the autumn months. Tides varied from 1-2 feet in most places, but could be as much as nine feet in the mouth of the Gilrain-Serni bay.