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Markha
Markha (Марха) is a 1,181-km river in Yakutia, Russia, a left tributary of the Vilyuy in the Lena basin, rising on the Vilyuy Plateau. Its basin holds Yakutia's diamond fields and the Kraton-3 (1978) nuclear explosion site; not the Ladakh Markha River.
Marlborough Sounds
Marlborough Sounds, New Zealand drowned river valley network (rias) at the top of the South Island, including Queen Charlotte Sound/Tōtaranui and Pelorus/Te Hoiere; about 4,000 km², one tenth of NZ's coastline, formed by Miocene incision, subsidence, and sea-level rise.
Marne (river)
The Marne is a 525 km river in northern France rising on the Langres plateau and the Seine's main tributary, which it joins at Charenton-le-Pont upstream of Paris.
Marsh
Marsh: a wetland frequently or continually inundated with water, dominated by emergent soft-stemmed vegetation such as cattails and cordgrass; distinct from swamps (woody), bogs (acidic peat), and fens (groundwater-fed), with neutral pH, high nutrients, and flood- and storm-buffering functions.
Marsh Island
Marsh Island, Louisiana: a ~71,000-acre brackish marsh island in Iberia Parish between Vermilion Bay and the Gulf, donated by Margaret Sage in 1920 and managed by LDWF as a wildlife refuge.
Martimoaapa - Lumiaapa - Penikat Mires
Martimoaapa-Lumiaapa-Penikat Mires, also called Martimoaapa mire reserve, is a 14,086 ha Ramsar site no. 11 and Natura 2000 SAC/SPA in Simo and Keminmaa, Sea Lapland, Finland, designated in 1974, established as a mire reserve in 1981, with nearly 100 nesting bird species.
Martre
Lac la Martre (Martin's Lake) is a 1,776 km² shallow glacial lake in the Northwest Territories at Whatì, draining via the La Martre River (14,500 km² watershed) to Great Slave Lake; its streamflow has risen more than any other gauged NWT catchment.
Mascarene Basin
Mascarene Basin, ocean basin of the western Indian Ocean east of Madagascar; extinct spreading ridge active about 84–60 Ma between India and Madagascar, succeeded by the Carlsberg Ridge.
Mascarene Plateau
The Mascarene Plateau is an arcuate submerged seafloor high in the southwestern Indian Ocean, extending over 2,300 km from the Seychelles to Mauritius, with shallow banks hosting the world's largest contiguous seagrass beds.
Mason Inlet
Mason Inlet is an Antarctic inlet on the Black Coast, Palmer Land, between Cape Herdman and Cape Mackintosh at 72°53'S 60°15'W, named for FIDS surveyor Douglas Percy Mason and fed by Clowes and Watkins glaciers.
Massachusetts Bay
Massachusetts Bay is an Atlantic bay at the southern end of the Gulf of Maine between Cape Ann and Cape Cod, namesake of the 1628 Massachusetts Bay Company and Colony, with semidiurnal tides, Stellwagen Bank, and 12,000 years of Native habitation.
Matagorda Bay
Matagorda Bay is a large estuary on the central Texas Gulf coast, fed by the Lavaca and Colorado Rivers and sheltered from the Gulf by Matagorda Peninsula.
Matha Strait
Matha Strait (Estrecho Matha) is a deep, navigable NW-SE strait between Adelaide Island and the Biscoe Islands off the Antarctic Peninsula, named for André Matha of Charcot's expedition and renamed from Matha Bay by Rymill's 1934-37 BGLE survey.
Matochkin Strait
Matochkin Strait (Маточкин Шар, Matochkin Shar) is a 2–3 km wide strait separating Severny and Yuzhny islands of Novaya Zemlya, formed along a transverse fault, with Proterozoic basement slices, ca. 700 Ma granites, and nearby underground nuclear testing at Mt. Lasareff.
Maud Rise
Maud Rise, a large volcanic undersea plateau in the eastern Weddell Sea near 66°S 3°E, marked by a Taylor cap circulation, a warm-water Halo, and recurring open-ocean polynyas in the 1970s and 2016–2017.
Maxwell Bay
Maxwell Bay (Fildes Bay), a U-shaped fjord between King George and Nelson Islands, South Shetland Islands, Antarctica; named by Weddell after Francis Maxwell; hosts Chilean, Russian, Korean, and other research stations.
Maya (landform)
Maya (landform): chiefly the Maya River (Мая, маайа), a right tributary of the Aldan in Siberia, over 1,050 km long; also the Maya Mountains of Belize, Sudan's Mayas wetlands, and a Leyte stream.
Mayran - Viesca
Mayran - Viesca, FEOW ecoregion 163, is a closed-basin xeric freshwater ecoregion of north-central Mexico centered on the Nazas-Aguanaval drainage, known for endemic desert pupfishes, minnows, and suckers and two extinct Parras basin species.
Mbomou
Mbomou (Bomu) River, Central African river rising on the Sudan–CAR frontier, flowing west along the DRC border; joins the Uele to form the Ubangi, a major Congo tributary.
McFarlane Strait
McFarlane Strait: strait between Greenwich and Livingston Islands, South Shetland Islands, Antarctica, near 62°32′S 59°55′W; also called Yankee Sound; named for Captain Andrew McFarlane of the brig Dragon; strong tidal currents make passage hazardous.
McLeod Bay
McLeod Bay names several bays: an official bay on the British Columbia coast (CGNDB key JBKQU), a deep East Arm basin of Great Slave Lake, NWT, a bay on Dall Island, Alaska, and a bay on Gauer Lake, Manitoba.
McMurdo Sound
McMurdo Sound is the Antarctic sound between Ross Island and the mainland, discovered by James Clark Ross in 1841 and named for Lt. Archibald McMurdo. It hosts McMurdo Station and Scott Base, stable fast ice south of 77.6°S, polynyas to the north, and near-freezing waters around -2°C.
McWay Falls
McWay Falls, also called McWay Creek Falls, is an 80-foot waterfall in Julia Pfeiffer Burns State Park on the Big Sur coast that drops onto a beach in McWay Cove.
Meander
A meander is a bend in a river that grows more pronounced as water erodes the outside of the curve and deposits sediment on the inside.
Meander neck
Meander neck: the narrow land strip between the two limbs of a river meander bend. When migration narrows it below one channel width, a neck cutoff breaches it, forming an oxbow lake; distinct from chute cutoffs.
Mearim
Mearim (Rio Mearim): river in Maranhão, Brazil, rising on the Serra da Menina and flowing to São Marcos Bay; largest basin in the state, with a 646-km navigable waterway and tributaries Pindaré and Grajaú.
Measurement and disputed heights of waterfalls
Waterfall heights are measured by conventions such as total drop or tallest single drop, and survey quality explains why references disagree by hundreds of meters.
Medicine Lake Wetland Management District
Medicine Lake Wetland Management District, officially the Northeast Montana Wetland Management District, is a U.S. Fish and Wildlife Service unit protecting waterfowl habitat in northeastern Montana.
Mediterranean Ridge
Mediterranean Ridge: submarine accretionary wedge of the Hellenic subduction zone, where Africa subducts beneath the Aegean at about 40 mm/yr; over 300 km wide, 2,000 km long, fastest growing accretionary complex on Earth, with abundant mud volcanoes and Messinian salt diapirs.
Mekong (湄公河)
The Mekong (湄公河), called Lancang in China, is a roughly 4,900 km river flowing through six countries to the Mekong Delta, supporting the world's largest inland fishery and a basin reshaped by dams, sediment loss and mining pollution.