2,436 articles
Opening of the Atlantic Ocean
The opening of the Atlantic Ocean began as Pangaea rifted apart about 201 million years ago, with Central Atlantic seafloor spreading starting around 190 Ma.
Operation Highjump
Operation Highjump (officially the U.S. Navy Antarctic Developments Program, 1946-47), Task Force 68 under Byrd and Cruzen: 4,700 men, 13 ships, aerial photomapping of Antarctica, Little America IV, early withdrawal from Ross Sea pack ice.
Orange River
The Orange River (Senqu in Lesotho) rises in Lesotho's Maluti Mountains, flows through South Africa along the Namibia border to the Atlantic, shared by four states and managed by ORASECOM since 2000.
Oregon & Northern California Coastal
Oregon & Northern California Coastal is a freshwater ecoregion, FEOW ID 123, covering Pacific coast drainages from Oregon to San Francisco Bay, with about a quarter of its fish species endemic.
Oregon Lakes
Oregon Lakes: FEOW freshwater ecoregion 124, Nearctic, xeric endorheic basins; also Oregon's 1,400+ named lakes, including Crater Lake (deepest in US, 1,932 ft) and Upper Klamath Lake.
Øresund
Øresund (the Sound) is the 118 km strait between Denmark and Sweden linking the Baltic Sea and Kattegat, site of the Sound Toll (c. 1429–1857) and the 2000 Øresund Bridge between Copenhagen and Malmö.
Orinoco
Orinoco: major South American river draining 1,212,000 km² of Colombia and Venezuela, discharging 33,000 m³/s (third largest worldwide), linked to the Amazon via the Casiquiare, with 1,002 fish species.
Orinoco Delta & Coastal Drainages
Orinoco Delta & Coastal Drainages, FEOW freshwater ecoregion 309, covers the Orinoco Delta in Venezuela plus coastal rivers of northwestern Guyana; a 22,000 km2 tidal wetland with four endemic fishes, the Warao people, and the Delta del Orinoco Biosphere Reserve.
Orinoco Guiana Shield
Orinoco Guiana Shield, FEOW freshwater ecoregion 308, is the clear- and blackwater upland half of the Orinoco basin in Venezuela, drained by the Ventuari, Caura, and Caroní, with tepuis to 2800 m, 49 endemic fishes, and mercury-releasing gold mining as its main threat.
Orinoco High Andes
Orinoco High Andes, FEOW freshwater ecoregion 305, montane freshwaters in the Colombian and Venezuelan Andes draining to the Orinoco; headwaters of the Guaviare, Meta, Arauca, and Apure rivers, six endemic fishes, highest point Sierra Nevada del Cocuy at 5493 m.
Orinoco Piedmont
Orinoco Piedmont, FEOW freshwater ecoregion 306, a montane transition between the Orinoco High Andes and Llanos in Colombia and Venezuela; upper Guaviare, Meta, Arauca, Apure rivers; nine endemic fish species; threatened by deforestation and oil.
Orkhon
Orkhon (Орхон) is Mongolia's longest river, 1,124 km, rising in the Khangai Mountains and joining the Selenga, which feeds Lake Baikal. Its valley holds Kharakhorum, Khar Balgas, and the UNESCO Orkhon Valley Cultural Landscape.
Orléans Strait
Orléans Strait (Canal d'Orléans, Dallmannstraße) is a NE-SW Antarctic Peninsula strait separating Trinity and Tower islands from Davis Coast; named by d'Urville's 1837-40 French expedition, charted by Nordenskjöld.
Orontes
Orontes (Nahr al-Assi, Asi River), a Middle East river rising in Lebanon and flowing through Syria and Turkey to the Mediterranean; also the name of Achaemenid satraps of Armenia and the Orontid dynasty.
Osage River
Osage River: Kansas-Missouri tributary of the Missouri River, rising as the Marais des Cygnes, nearly 500 miles long, dammed by Bagnell Dam (1931, Lake of the Ozarks) and Truman Dam.
OSPAR Convention
OSPAR Convention (Convention for the Protection of the Marine Environment of the North-East Atlantic), signed 1992, in force 1998; replaces Oslo 1972 and Paris 1974 conventions; 15 governments plus EU governed by the OSPAR Commission through binding Decisions and non-binding Recommendations.
Ostrov Oleniy
Ostrov Oleniy (Остров Олений, "Deer Island") is the name of several separate islands in Russia, the largest lying in the southern Kara Sea off Siberia's Gyda Peninsula.
Oti
The Oti River, called the Pendjari in Benin, is a West African river that rises in the Atakora Mountains and flows through Togo and Ghana into the Volta, its largest tributary.
Otsego Lake (New York)
Otsego Lake is a deep glacial lake in Otsego County, New York, directly north of Cooperstown and the source of the Susquehanna River; James Fenimore Cooper called it "Glimmerglass."
Overfalls
Overfalls are areas of rough, breaking sea caused by strong tidal currents over shoals, meeting currents, or wind against tide; a charted nautical hazard related to tide rips and tidal races.
Owens River
Owens River, eastern California watercourse rising near Mount Lyell and flowing to Owens Lake; since 1913 diverted into the Los Angeles Aqueduct, drying the lake and driving dust mitigation and the Lower Owens River Project restoration.
Oxbow lake
Oxbow lake: crescent-shaped lake on a floodplain formed when a meandering river cuts off a loop; fills with sediment over centuries, leaving a clay plug.
Ozero Srednye Kuyto
Ozero Srednye Kuyto (Keski-Kuittijärvi), the largest of Karelia's three Kuyto lakes in the Kem River basin: 275.7 km², 41.8 km long, 34 m deep, reservoir since 1956, with Kalevala on its northern shore.
Pacific decadal oscillation
Pacific decadal oscillation (PDO), related to the Interdecadal Pacific Oscillation (IPO), is a decadal-scale pattern of North Pacific sea surface temperature and pressure variability, defined by Mantua et al. 1997, with warm and cool phases affecting North American climate and fisheries.
Pacific hurricane
Pacific hurricane: a tropical cyclone in the eastern or central North Pacific reaching 74 mph winds; season May 15 to November 30, averaging 15 named storms, 8 hurricanes, 4 major hurricanes, boosted by El Niño.
Pacific Ocean
The Pacific Ocean, Earth's largest ocean, lies between the Americas and Asia-Australia, divided into North and South Pacific by convention. Key facts: Bering Strait is its only Arctic gateway (~1 Sv flow, rising since 1990), and IHO limits place its southern boundary at 60°S.
Pacific-Antarctic Ridge
Pacific-Antarctic Ridge: South Pacific mid-ocean ridge spreading center between the Pacific and Antarctic plates, with en echelon segments, a mantle boundary near the Easter microplate, and interaction with the Foundation hotspot.
Pactolus Bank
Pactolus Bank, also called Burnham Bank, is a reported submerged bank in the Southern Ocean west of Cape Horn, sounded in 1885 and never confirmed.
Päijänne
Päijänne, Finland's second largest and deepest lake (95 m), a 120 km oligotrophic watercourse in central Finland; source of Helsinki's drinking water via the 120 km Päijänne Tunnel, completed 1982, with Päijänne National Park on its southern basins.
Palk Strait
Palk Strait (Paakku Niirinai, Pok Samudra Sandhiya) is the shallow 75-km-wide strait between Tamil Nadu, India, and northern Sri Lanka, named after Governor Sir Robert Palk, bounded by Adam's Bridge and central to the India–Sri Lanka trawling dispute.