The Nahanni National Park Reserve, sometimes known as "Headless Valley" or "Valley of The Headless Men" (after a series of unsolved historical deaths in the park), in the Dehcho Region of the Northwest Territories, Canada (approximately 500 km (311 mi) west of Yellowknife),[4] protects a portion of the Mackenzie Mountains Natural Region.

[6]: 87  The park was one of the earliest locations to be inscribed as World Heritage Sites by UNESCO; this designation was given in 1978 due to the area's picturesque wilderness, its various waterways and giant falls, and its 1,000 meter-deep limestone canyons, among other features.

Sediments, left by an ancient inland sea present some 500–200 million years ago, have since become pressed into coloured layers of rock.

These layers, stacked about 6 km (3.7 mi) deep, are peppered with fossilised remnants of the varied organisms that once swam the ancient seas, beginning well before the time of the dinosaurs.

While there are no active volcanoes in the park, towers of heated rock (igneous batholiths) were sent upwards, pushing the sediment further up.

The top layer of sedimentary rock was eventually worn-away, resulting in granite towers that form the Ragged Range.

Slightly further north lies Mount Sir James MacBrien (62°07′23″N 127°40′47″W / 62.12306°N 127.67972°W / 62.12306; -127.67972 (Mount Sir James MacBrien)), the territories second highest peak at 2,759 m (9,052 ft), and Lotus Flower Tower (2,570 m (8,430 ft), 62°06′51.6″N 127°41′50.4″W / 62.114333°N 127.697333°W / 62.114333; -127.697333 (Lotus Flower Tower)) both of which form part of the Cirque of the Unclimbables (62°06′06″N 127°40′15″W / 62.10167°N 127.67083°W / 62.10167; -127.67083 (Cirque of the Unclimbables)).

[9] The falls were initially located downstream at the east end of Fourth Canyon, but, over the centuries, have carved their way backwards through the limestone that lines the river.

[6]: 16  Due to the constant mists created by the waterfalls, and the insulating cliff walls, a favourable microclimate in the immediate vicinity of the falls fosters a rich botanical biodiversity, including several boreal orchid species, such as Calypso bulbosa, Cypripedium parviflorum and passerinum, Galearis rotundifolia, and Platanthera aquilonis, huronensis and obtusata.

[6]: 14  The source of the springs comes from deep in the Earth's crust, near the base of the granite batholiths that form the Ragged Range.

This process takes a great deal of time, and it is believed that the mounds themselves are around 10,000 years old, their creation beginning at the end of the last ice age.

[12] The park's sulphur hot springs, alpine tundra, mountain ranges, and forests of spruce and aspen are home to many species of birds, fish and mammals.

These include the common nighthawk, grizzly bear, olive-sided flycatcher, peregrine falcon, rusty blackbird, short-eared owl, wood bison, woodland caribou, wolverine and yellow rail.

[14]: 14 Mammal species found in the park include the American black bear, American mink, beaver, Arctic ground squirrel, Canada lynx, collared pika, Dall sheep, fisher, grey wolf, hoary bat, hoary marmot, least chipmunk, least weasel, moose, muskrat, northern myotis, mule deer, pine marten, red fox, red squirrel, river otter, Rocky Mountain goat, snowshoe hare, white-tailed deer and several types of shrew and vole.

[13] Birds are numerous, and include American kestrel, bald and golden eagles, barred owl, belted kingfisher, boreal owl, black-capped and boreal chickadees, Canada goose, Canada jay, eared grebe, great grey owl, great horned owl, grouse, gulls, gyrfalcon, harlequin ducks, horned grebe, loons (the common, Pacific, red-throated and yellow-billed), mallards, northern harrier, northern shrike, osprey, pied-billed grebe, ptarmigan, redpoll, red-necked grebe, red phalarope, sandpipers and plovers, sharp-shinned hawk, snow goose, surf scoter, swallows, woodpeckers and sapsuckers, and trumpeter and tundra swans.

[6]: 30 The Dene, sometimes called Slavey peoples, have used the lands around Nahanni National Park Reserve for thousands of years.

The local oral history contains many references to the Naha tribe, a mountain-dwelling people who used to raid settlements in the adjacent lowlands.

[16] During the 19th century, most Dene families left their nomadic lifestyles and settled into more permanent communities, often close to the trading posts.

Constructed from six to ten untanned moose hides sewn together and stretched over a spruce pole frame, these boats would transport entire families, their dogs and cargo of furs down the river during high water.

[18] In 1946 Calgary geologist and mining expert Frank M. W. Henderson returned from the valley reporting his partner John Patterson had disappeared.

Henderson and his party camped there a few days, but one night were awoken by a group of First Nations people who warned of white figures moving along the valley.

'"[20] In 1964, explorer parachutist Jean Poirel from Montreal jumped at its source 500 km (310 mi) north of Yellowknife, followed by his teammate Bertrand Bordet.

The most important contained 116 Dall sheep's skeletons (carbon-14 dated to 2500 years BC); Jean Poirel named it "Valerie Cavern" after his daughter.

[21][22] Originally established in 1972, by then Prime Minister Pierre Elliot Trudeau,[4] the park was 4,766 km2 (1,840 sq mi) in area.

However, only Virginia Falls and Glacier Lake are designated for day use visitation, meaning all other sites require visitors to stay overnight in the park.

The Nahanni River and its surroundings has also been featured in the French book La Vallée Sans Homme, written in 1973 by Roger Frison-Roche.