River ice begins to break up at Whitehorse, the territorial capital, in early May and loosens northward on Lake Laberge a few days later. The snow-fed tributaries from the Mackenzie Mountains to the east reach their flood peak in June, but glacier-fed streams such as the White, draining the north slopes of the St. Elias Mountains, do not have their greatest runoff until July. Once these peak runoff flows have passed, the Yukon River becomes a shallow, braided stream, as summer precipitation in the Yukon basin is low—about 6 inches (150 millimetres) of summer rainfall is recorded at Whitehorse and Dawson.

North of Lake Laberge, the Yukon flows through a broad, generally flat-bottomed valley; riverbanks are not high, and the river meanders lazily across the valley floor, with numerous sandbars and small islands dotting the channel. About 25 miles north of the village of Carmacks, four small, rocky islands split the river into the Five Finger Rapid area, and the current flows swiftly past these islands. At the village of Selkirk, the junction of the Pelly River swells the Yukon's volume considerably, for the Pelly drains about 19,700 square miles of the generally uninhabited western slopes of the Mackenzie Mountains. North of Selkirk, the Yukon is a clear and gentle river, with numerous low islands breaking the channel. The White River, with a drainage area of about 19,500 square miles, adds silt from the glaciers and mountains to the southwest. The Stewart River, having about the same drainage area as the White River, flows out of the mining area of Mayo-Keno City to the east. At Dawson the Yukon has an average flow of 74,000 cubic feet (2,095 cubic metres) per second, but there is a wide difference between the high-water flows of June and the low water beneath the ice in March. There are no hydroelectric power developments on the Yukon, except for a small plant at Whitehorse. The river is thought to have considerable generating potential; but, because there has been no demand for power, no accurate evaluations have been made.

The Klondike is a small, east-bank tributary that joins the Yukon at the historic territorial town of Dawson. The Klondike valley floor is covered with sinuous, wormlike ridges of gravel deposited behind the gold dredges that have worked their way through the valley gravels over a period of nearly a century.

Downstream from Dawson the river valley narrows, and the low plateau is broken by mountains that rise to elevations of more than 4,000 feet, about 15 miles north of the Yukon River. The river itself is about 1,000 feet above sea level before it crosses the border into Alaska. The next large tributary, the Porcupine River, joins the Yukon at Fort Yukon, Alaska; the Porcupine drains the northern Yukon territory and the southeastern slope of the high and rugged Brooks Range in Alaska, a land occupied by a small population of Indians. The Yukon then flows generally westward for about 150 miles across a broad, flat valley; the many channels of the braided river thread their way through numerous islands and sandbars. At the western end of the “flats” is a narrow gorge, where the river rushes through a low mountain barrier known as the ramparts.

At the junction of the Tanana River, the main southern tributary in Alaska, the Yukon is less than 300 feet above sea level. Upstream on the Tanana lies Fairbanks, the largest city in the Yukon River basin. The snow- and glacier-fed waters of the Tanana drain the north slopes of the high Alaska Range, but, after the peak runoff in July, the Tanana becomes a shallow stream by late summer. West of the Yukon's confluence with the Tanana, the Yukon valley broadens south of the river; but within 5 miles of its northern banks rise the low Kokrines Hills, with peaks over 3,000 feet in altitude. About 175 miles downstream from the Tanana confluence, the last major tributary, the Koyukuk, drains southward from the south-central slopes of the Brooks Range. The Yukon there is only 117 feet above sea level and only about 100 miles due east of Norton Sound, but the river is forced to detour several hundred miles to the south and west around mountainous terrain.

As the Yukon nears the Bering Sea, it bends sharply northward to empty into Norton Sound. The river delta is about 40 miles across, swampy, and lake-covered. Many of the distributary channels are shallow, but the northeastern one, Apoon Pass, was once used by riverboats.

The valleys of the Yukon River basin, where nearly all of the population lives, have a subarctic climate with relatively warm, short summers; but the treeless upper mountain slopes, where there are no weather stations, are classified as having an Arctic climate. Bitterly cold air masses from Siberia or the Arctic Ocean cross Alaska and the Yukon territory throughout the long, dark winter, but occasional warm air masses from the Pacific Ocean can penetrate over the high barrier of the St. Elias Mountains and bring warmer winter temperatures to the Whitehorse area. January mean temperatures are about -24° F (-31° C) at Dawson and about -6° F (-21° C) at Whitehorse. In 1947 the lowest temperature officially recorded in North America, -81° F (-63° C), was reported at Snag along the Alaska Highway in southwestern Yukon.

Average summer temperatures in the valleys of this high-latitude region, with its long hours of summer daylight, are moderately warm, with July mean temperatures of about 60° F (16° C) at Dawson and slightly lower at Whitehorse. Annual precipitation is low, as it is in most of northwestern Canada and central Alaska, with an average of about 10 inches (260 millimetres) recorded at Whitehorse and 12 inches at Dawson. Almost 50 percent of the precipitation falls as rain during the four summer months.

Coniferous forests grow to low height and low densities across the valley floors and lower mountain slopes of the Yukon River basin. The upper tree line is at an elevation of about 3,500 to 4,500 feet, depending on local exposure and drainage; thus, about two-thirds of the mountainous Yukon territory is treeless. The largest stands of potentially commercial forest are along the Alaska Highway in the Liard River valley in the southeastern Yukon, but this area is far from markets. White spruce is dominant on the valley floors, and black spruce grows in the poorly drained areas. Outbreaks of spruce budworm threaten large areas of these forests. In addition, Alpine fir and lodgepole pine are found in the lower areas along the Alaska Highway across the southern Yukon.

A varied animal life inhabits the forested valleys. The larger mammals include black, brown, and grizzly bears; caribou, deer, and moose; and mountain goats and sheep at higher elevations. Timber wolves are common. Game birds such as grouse and ptarmigan are found, and waterfowl include many species of geese, swans, and ducks. The usual furbearers trapped by the Indian population include muskrat, mink, marten, lynx, weasel, fox, fisher, and squirrel. In the river itself are found species of such fish as Arctic grayling, burbot, pike, salmon, and whitefish.

The Yukon River basin has remained sparsely populated in the century and a half since it was first settled by Europeans. The lure of mineral wealth has been the main attraction of the region, and mining has maintained several of the settlements. Gold brought people to Fairbanks and Dawson; when the gold was depleted, Fairbanks continued to grow as it took on administrative and transportation functions for east-central Alaska, but the population of Dawson declined to only a tiny fraction of its size in the heyday of the gold rush. In the 1950s the Yukon territorial capital was moved from Dawson to the more accessible city of Whitehorse, on the Alaska Highway. Whitehorse then developed transportation and service functions for the other small settlements in the southern Yukon region and became the largest city in the territory.

Mines are marginal economic operations in the Yukon River basin because the area is far from world markets and has only a few land-transport routes. Lead, zinc, and silver have been produced at Keno City, near Mayo, since the 1920s, but the high-grade ore has to be transported southward to Trail, B.C., near the U.S.-Canadian border, for smelting. Since the early 1970s, copper and base metals have been mined at Faro, on the Pelly River west of the Mackenzie Mountains, and ore concentrates have been transported by truck to Whitehorse. Some gold is still extracted intermittently from alluvial deposits in several Yukon tributaries, and deposits of tungsten, iron, and coal are known.

Other economic activities are of minor importance in the Yukon basin. Fur trapping has been the traditional livelihood for the region's Indians, but low monetary returns plus the attraction of better pay from wage labour in the towns have taken many Indians away from their former migratory lives. Agriculture is a minor activity. Although vegetables, pasturage, and coarse grains can be grown during the long days of the short summer, only a small number of operating farms exist; most of the food consumed in Whitehorse, for example, is shipped in from southern Canadian farms. Perhaps the main resource of the Yukon basin is its scenery, isolation, and sparse population, all of which are attractive to tourists seeking to escape the more crowded and less scenically endowed areas of the continent. The many vistas of the winding river, its forested slopes supporting game and wildlife, and the game fish in the tributary lakes are all part of an attractive natural environment that has become more difficult to find in southern regions.

The delta mouth of the Yukon River was known to the Russians when they occupied nearby St. Michael Island in 1831, but the headwaters in British North America remained unknown for another decade. By 1838 Russian fur traders had explored the river as far inland as Nulato (Alaska), where they established a post near the junction of Koyukuk River. By 1846 the Russians had mapped almost 600 miles of the lower river. The trader Robert Campbell, of the Hudson's Bay Company, explored Pelly River, one of the Yukon headwaters, in 1840. In 1848 he established a trading post at Fort Selkirk, at the junction of the Pelly and Yukon rivers, in order to trade with the local Indians. In 1851 Campbell explored the Yukon downstream to the junction of the Porcupine River, where Fort Yukon had been built in 1847. When the trading post at Fort Selkirk was burned by hostile Indians in 1852, however, Europeans withdrew from the upper Yukon basin for two decades.