710 articles
Ohio River
The Ohio River, called O-Y-O ("the great river") by the Iroquois, is a 981-mile tributary of the Mississippi formed at Pittsburgh by the Allegheny and Monongahela Rivers, ending at Cairo, Illinois, with 20 navigation dams and over 230 million tons of annual cargo.
Oka (اوكا)
Oka (Ока), river in European Russia, largest right tributary of the Volga; 1,500 km, 245,000 km² basin, rises in Oryol Oblast, joins the Volga at Nizhny Novgorod; heavily polluted in middle reaches.
Okavango (river, Botswana)
The Okavango River, called the Cubango in Angola, is a southwestern African river about 1,600 km long that rises on the Bie Plateau and ends in Botswana's inland Okavango Delta.
Okhotsk Coast
Okhotsk Coast (FEOW ecoregion 614): Russian Far East freshwater ecoregion of short coastal rivers draining the Sea of Okhotsk coast from Gizhiginskaya Guba Bay to the Kiran River, marked by charr diversity and absence of Coregonidae and Amur fauna.
Olenyok
Olenyok (Оленёк), a 2,270 km river in Yakutia and Krasnoyarsk Krai, Russia, flowing from the Vilyuy Plateau to Olenyok Bay of the Laptev Sea; discharge 1,210 m³/s, basin 219,000 km², site of the 1633 Olenyok winter quarters.
Olyokma
Rising on the northern slope of the Yablonovy Range, the Olyokma (Олёкма, Olekma) is a right tributary of the Lena and flows through Amur Oblast and Yakutia.
Omolon
Omolon (Омолон), river in Magadan Oblast, Chukotka, and Yakutia, Russia; longest right tributary of the Kolyma, 1,114 km, basin ~113,000 km², navigable ~595 km, village of Omolon on its banks.
Onon
Onon (Онон гол, 鄂嫩河, historic 斡難河) is a 1,032 km river rising in Mongolia's Khentii mountains, joining the Ingoda to form the Shilka of the Amur basin; birthplace region of Genghis Khan.
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.
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 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 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.
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.
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.
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.
Panch Prayag
Panch Prayag, the five sacred confluences of the Alaknanda River in Garhwal, Uttarakhand: Vishnuprayag, Nandaprayag, Karnaprayag, Rudraprayag, and Devprayag, where the Ganga is formed.
Pangani
Pangani: river, basin, and freshwater ecoregion (FEOW 568) in northeastern Tanzania and Kenya, rising on Kilimanjaro and Meru, flowing 500 km to the Indian Ocean; noted for endemic fish, hydropower, and severe water stress.
Panj
Panj (Панҷ), also Pyanj or Pyandzh: 921-km Central Asian river rising in the Pamirs, forming much of the Afghanistan-Tajikistan border and feeding the Amu Darya; subject of Tajik-Afghan hydrology cooperation, flood hazards, and glacier-fed flow.
Panuco
Panuco (Pánuco), FEOW freshwater ecoregion 167, a tropical and subtropical coastal-river ecoregion on Mexico's Gulf slope centered on the Río Pánuco basin, noted for roughly 60% endemic fishes, Xiphophorus and Herichthys radiations, and endangered conservation status.
Papaloapan
Papaloapan: major river of Mexico's southern Gulf slope, draining a ~46,517 km² basin across Oaxaca, Veracruz, and Puebla; second by flow after Grijalva-Usumacinta; site of the 1947 Papaloapan Commission, Miguel Alemán and Cerro de Oro dams, and a highly endemic freshwater fish fauna.
Paraná River
Paraná River (Rio Paraná), ninth largest river by discharge, flows through a basin spanning Argentina, Brazil, Bolivia, and Paraguay; heavily dammed (Itaipú, Yacyretá, Porto Primavera), navigated since 1865, hit by 2021 drought emergencies.
Paranaíba
Paranaíba River, also Rio Paranaíba, a 1,120 km Brazilian river rising in Serra da Mata da Corda, Minas Gerais, at 1,140 m; joins the Rio Grande to form the Paraná; basin of 222,767 km² with dams Itumbiara, Emborcação, Nova Ponte; distinct from the Parnaíba River and the city of Paranaíba (MS).
Paranapanema
Paranapanema (rio Paranapanema), a left-bank tributary of the Paraná River on the São Paulo–Paraná border, Brazil; 106,554.53 km² basin, 11 hydropower dams including Chavantes and Capivara, 225 fish species.
Parnaíba (landform)
The Parnaíba (Rio Parnaíba) is a river of northeastern Brazil that flows about 1,500 km to the Atlantic, where it forms the Parnaíba Delta on the Piauí–Maranhão border.
Patapsco River
The Patapsco River is a major Maryland river that flows generally southeast into the Chesapeake Bay. Its valley is known as the cradle of the state's Industrial Revolution.
Peace River
The Peace River in western Canada flows from the Rocky Mountains in British Columbia through Alberta to the Peace-Athabasca Delta. It is named for a Beaver-Cree peace settlement and hosts three hydroelectric dams including Site C (1,100 MW).
Pearl (landform)
Pearl (landform): chiefly the Pearl River of Mississippi and Louisiana, a 400+ mile Gulf-draining river named for 1698 pearl finds, and China's Pearl River (Zhu Jiang), formed by the West, North, and East Rivers.