Question: When did the ocean that the Murray Mouth flows into become a thing?

full_context: Document 1: Murray River: The Murray River (or River Murray) (Ngarrindjeri: Millewa, Yorta Yorta: Tongala) is Australia's longest river, at 2,508 kilometres (1,558 mi) in length. The Murray rises in the Australian Alps, draining the western side of Australia's highest mountains, and then meanders across Australia's inland plains, forming the border between the states of New South Wales and Victoria as it flows to the northwest into South Australia. It turns south at Morgan for its final 315 kilometres (196 mi), reaching the ocean at Lake Alexandrina.
Document 2: Borders of the oceans: The Southern Ocean did not appear in the 1953 third edition because ``... the northern limits... are difficult to lay down owing to their seasonal change... Hydrographic Offices who issue separate publications dealing with this area are therefore left to decide their own northern limits. (Great Britain uses the Latitude of 55 ° South) ''. Instead, in the IHO 1953 publication, the Atlantic, Indian and Pacific Oceans were extended southward, the Indian and Pacific Oceans (which had not previously touched pre 1953, as per the first and second editions) now abutted at the meridian of South East Cape, and the southern limits of the Great Australian Bight and the Tasman Sea were moved northwards.
Document 3: Antarctica: Positioned asymmetrically around the South Pole and largely south of the Antarctic Circle, Antarctica is the southernmost continent and is surrounded by the Southern Ocean; alternatively, it may be considered to be surrounded by the southern Pacific, Atlantic, and Indian Oceans, or by the southern waters of the World Ocean. It covers more than 14,000,000 km2 (5,400,000 sq mi), making it the fifth-largest continent, about 1.3 times as large as Europe. The coastline measures 17,968 km (11,165 mi) and is mostly characterized by ice formations, as the following table shows:
Document 4: Amundsen Sea: The Amundsen Sea, an arm of the Southern Ocean off Marie Byrd Land in western Antarctica, lies between Cape Flying Fish (the northwestern tip of Thurston Island) to the east and Cape Dart on Siple Island to the west. Cape Flying Fish marks the boundary between the Amundsen Sea and the Bellingshausen Sea. West of Cape Dart there is no named marginal sea of the Southern Ocean between the Amundsen and Ross Seas. The Norwegian expedition of 1928–1929 under Captain Nils Larsen named the body of water for the Norwegian polar explorer Roald Amundsen while exploring this area in February 1929.
Document 5: Capel River: An artificial river mouth was cut through the sand dunes adjacent to the Stirling Wetlands in 1880 to allow it to flow directly into Geographe Bay.
Document 6: Ceyhan River: In classical times the Ceyhan River reached the Mediterranean Sea at Mallus. Currently this location is inland a few miles from the Mediterranean coast on an elevation in the Karataş Peninsula, Adana Province, Turkey, a few miles from the town of Karataş. In classical times it carried with it such a quantity of mud, that, according to an ancient oracle, its deposits would one day reach the island of Cyprus, and thus unite it with the mainland. The delta formed at the mouth of the Ceyhan River is a refuge for wetlands birds and some years their numbers reach a few million. The sand bank in front is famed for nesting ground for sea turtles.
Document 7: Cann River: The Cann River rises southwest of Granite Mountain in remote country on the eastern boundary of the Errinundra National Park and flows generally east, then south, then east, then south through the western edge of the Coopracambra National Park and through the Croajingolong National Park, joined by seventeen minor tributaries before reaching its mouth with Bass Strait, at the Tamboon Inlet in the Shire of East Gippsland. The river descends over its course.
Document 8: Mokihinui River: The Mokihinui River's headwaters are located in the Glasgow Range and its mouth is on the Tasman Sea. There is little human habitation near the river: the localities of Mokihinui and Summerlea are near the river's mouth, Seddonville is a few kilometres up the river, and just prior to its terminus, State Highway 67 crosses the river outside Mokihinui. In the rugged back country behind Seddonville at the Mokihinui Forks, the river splits into two branches, north and south. The catchment of these two branches is a large inland basin of almost wholly unmodified forest.
Document 9: Cooperation Sea: The Cooperation Sea was named in 1962 by the Soviet Antarctic Expedition in honor of international science cooperation in Antarctica. The name first appeared as a proposal to the IHO in the IHO 2002 draft. This draft was never approved by the IHO (or any other organization), and the 1953 IHO document (which does not contain the name) remains currently in force. Leading geographic authorities and atlases do not use the name, including the 2014 10th edition of the World Atlas from the United States' National Geographic Society and the 2014 12th edition of the British Times Atlas of the World. But Soviet and Russian-issued maps do.
Document 10: Murray Mouth: Murray Mouth is the point at which the River Murray meets the Southern Ocean. The Murray Mouth's location is changeable. Historical records show that the channel out to sea moves along the sand dunes over time. At times of greater river flow and rough seas, the two bodies of water would erode the sand dunes to create a new channel leaving the old one to silt and disappear.
Document 11: Port River: The Port River is the western branch of the largest tidal estuary on the eastern side of Gulf St Vincent. It extends inland through the historic Inner Harbour of Port Adelaide, to the constructed salt-water West Lakes in the north-western suburbs of Adelaide. The lower reaches of the Port River flow between the Lefevre Peninsula, and the "Section Bank" and Torrens Island, and form the sea entrance to the port facilities of Adelaide, and connect to the Barker Inlet to the east via the "North Arm" and "Angas Inlet" which surround Garden Island. Before European settlement of Adelaide's western suburbs and the construction of various flood mitigation channels and levees, the Port River formed one of the outlets of the River Torrens.
Document 12: Rhine: The hydrography of the current delta is characterized by the delta's main arms, disconnected arms (Hollandse IJssel, Linge, Vecht, etc.) and smaller rivers and streams. Many rivers have been closed ("dammed") and now serve as drainage channels for the numerous polders. The construction of Delta Works changed the Delta in the second half of the 20th Century fundamentally. Currently Rhine water runs into the sea, or into former marine bays now separated from the sea, in five places, namely at the mouths of the Nieuwe Merwede, Nieuwe Waterway (Nieuwe Maas), Dordtse Kil, Spui and IJssel.
Document 13: Cape of Good Hope: A common misconception is that the Cape of Good Hope is the southern tip of Africa. This misconception was based on the misbelief that the Cape was the dividing point between the Atlantic and Indian Oceans. Contemporary geographic knowledge instead states the southernmost point of Africa is Cape Agulhas about 150 kilometres (90 mi) to the east - southeast. The currents of the two oceans meet at the point where the warm - water Agulhas current meets the cold water Benguela current and turns back on itself. That oceanic meeting point fluctuates between Cape Agulhas and Cape Point (about 1.2 kilometres (0.75 mi) east of the Cape of Good Hope).
Document 14: Southern Ocean: The Southern Ocean, geologically the youngest of the oceans, was formed when Antarctica and South America moved apart, opening the Drake Passage, roughly 30 million years ago. The separation of the continents allowed the formation of the Antarctic Circumpolar Current.
Document 15: Southern Ocean: By way of his voyages in the 1770s, Captain James Cook proved that waters encompassed the southern latitudes of the globe. Since then, geographers have disagreed on the Southern Ocean's northern boundary or even existence, considering the waters as various parts of the Pacific, Atlantic, and Indian Oceans, instead. However, according to Commodore John Leech of the International Hydrographic Organization (IHO), this situation has no scientific sense since it has different ecosystem and life from the three oceans mentioned before and is located on a southern portion of Earth, hence the name of an ocean. This remains the current official policy of the IHO, since a 2000 revision of its definitions including the Southern Ocean as the waters south of the 60th parallel has not yet been adopted. Others regard the seasonally - fluctuating Antarctic Convergence as the natural boundary.
ground_truth: the 1770s
sub_queries: What does Murray Mouth flow into? Answer: Southern Ocean
when did Southern Ocean become a thing Answer: the 1770s