The Ob proper is formed by the junction of the Biya and Katun rivers, in the foothills of the Siberian sector of the Altai, from which it has a course of 2,268 miles (3,650 km). If, however, the Irtysh River is regarded as part of the main course rather than as the Ob's major tributary, the maximum length, from the source of the Black (Chorny) Irtysh in China's sector of the Altai, is 3,362 miles (5,410 km), making the Ob the seventh longest river in the world. The catchment area is approximately 1,150,000 square miles (2,975,000 square km). Constituting about half of the drainage basin of the Kara Sea, the Ob's catchment area is the sixth largest in the world.

The West Siberian Plain covers about 85 percent of the Ob basin. The rest of the basin comprises the terraced plains of Turgay ( Kazakhstan) and the small hills of northernmost Kazakhstan in the south and the Kuznetsk Alatau range, the Salair Ridge, the Altai Mountains and their foothills and outliers in the southeast.

There are more than 1,900 rivers within the basin, with an aggregate length of about 112,000 miles (180,000 km). The Irtysh (Irtysh River), a left-bank tributary 2,640 miles (4,250 km) long, itself drains about 615,000 square miles (1,593,000 square km; a somewhat larger area than that drained by the upper and middle Ob above the Irtysh confluence); and some 70 percent of the whole basin is drained by left-bank tributaries.

The huge basin of the Ob stretches across a number of natural zones. Semidesert prevails in the far south around Lake Zaysan (Zaysan, Lake) (recipient of the Black Irtysh and source of the Irtysh proper), bordered on the north by steppe grassland. The central regions of the West Siberian Plain—i.e., more than half of the basin—consist of taiga (swampy coniferous forest), with great expanses of marshland. In the north there are vast stretches of tundra (low-lying, cold-tolerant vegetation).

The upper Ob runs from the junction of the Biya and Katun to the confluence of the Tom River, the middle Ob from the junction with the Tom to the Irtysh confluence, and the lower Ob from the junction with the Irtysh to the Gulf of Ob.

The Biya and the Katun both rise in the Altai Mountains: the former in Lake Telets, the latter to the south among the glaciers of Mount Belukha (Belukha, Mount). From their junction near Biysk the upper Ob at first flows westward, receiving the Peschanaya, Anuy, and Charysh rivers from the left; in this reach, the river has low banks of alluvium, a bed studded with islands and shoals, and an average gradient of 1 foot per mile (20 cm per km). From the Charysh confluence the upper Ob flows northward on its way to Barnaul, receiving another left-bank tributary, the Aley River, and widening its floodplain as the valley widens. Turning westward again at Barnaul, the river receives a right-bank tributary, the Chumysh River, from the Salair Ridge. The valley there is 3 to 6 miles (5 to 10 km) wide, with steeper ground on the left than on the right; the floodplain is extensive and characterized by diversionary branches of the river and by lakes; the bed is still full of shoals; and the gradient is reduced, but the depth increases markedly. At Kamen-na-Obi, however, where the river begins to bend northeastward, the width of the valley shrinks to 2 to 3 miles (3 to 5 km). Just above Novosibirsk another right-bank tributary, the Inya River, joins the upper Ob; and a dam at Novosibirsk forms the huge Novosibirsk Reservoir. Below Novosibirsk, where the river leaves the region of forest steppe to enter a zone of aspen and birch forest, both valley and floodplain broaden notably until, at the confluence with the Tom River, they are, respectively, 12 and 3 or more miles (19 and 5 or more km) wide. The depth of the upper Ob (at low water) varies between 6.5 and 20 feet (2 and 6 metres).

The middle Ob begins where the Tom flows into the main stream, from the right. Taking at first a northwesterly course, the river thereafter becomes much deeper and wider, especially after receiving its mightiest right-bank tributary, the Chulym (Chulym River), shortly below the confluence of the Shegarka River from the left. Successive tributaries along the northwesterly course, after the Chulym, include the Chaya and the Parabel (both left), the Ket (right), the Vasyugan (left), and the Tym and Vakh rivers (both right). Down to the Vasyugan confluence the river passes through the southern belt of the taiga, thereafter entering the middle belt. Below the Vakh confluence the middle Ob changes its course from northwesterly to westerly and receives more tributaries: the Tromyegan (right), the Great (Bolshoy) Yugan (left), the Lyamin (right), the Great Salym (left), the Nazym (right), and finally, at Khanty-Mansiysk, the Irtysh (Irtysh River) (left). In its course through the taiga, the middle Ob has a minimal gradient, a valley broadening to 18 to 30 miles (29 to 48 km) wide, and a correspondingly broadening floodplain—12 to 18 miles (19 to 29 km) wide. In this part of its course, the Ob flows in a complex network of channels, with the main bed widening from less than 1 mile (about 1 km) on the higher reaches to nearly 2 miles (3 km) at the confluence with the Irtysh and becoming progressively free of shoals. Low-water depths vary between 13 and 26 feet (4 and 8 metres). At high water there are great floods every year, sometimes spreading 15 or even 50 miles (24 to 80 km) across the valley and lasting from two to three months.

From its start at the confluence of the Irtysh, the lower Ob flows to the northwest as far as Peregrebnoye and thereafter to the north, crossing the northern belt of the taiga until it enters the zone of forest tundra in the vicinity of its delta. The valley is wide, with slopes steeper on the right than on the left, and the vast floodplain—12 to 18 miles (19 to 29 km) wide—is crisscrossed by the braided channels of the river and dotted with lakes. Below Peregrebnoye the river divides itself into two main channels: the Great (Bolshaya) Ob, which receives the Kazym and Kunovat rivers from the right, and the Little (Malaya) Ob, which receives the Northern (Severnaya) Sosva, the Vogulka, and the Synya rivers from the left. These main channels are reunited below Shuryshkary into a single stream that is up to 12 miles (19 km) wide and 130 feet (40 metres) deep; but after the confluence of the Poluy (from the right) the river branches out again to form a delta, the two principal arms of which are the Khamanelsk Ob, which receives the Shchuchya from the left, and the Nadym Ob, which is the more considerable of the pair. At the base of the delta lies the Gulf of Ob (Ob, Gulf of), which is some 500 miles (800 km) long and has a width reaching 50 miles (80 km); the gulf's own catchment area (forest tundra and tundra proper) is more than 40,000 square miles (105,000 square km).

The Ob basin has short, warm summers and long, cold winters. Average January temperatures range from −18 °F (−28 °C) on the shores of the Kara Sea to 3 °F (−16 °C) in the upper reaches of the Irtysh. July temperatures for the same locations, respectively, range from 40 °F (4 °C) to above 68 °F (20 °C). The absolute maximum temperature, in the arid south, is 104 °F (40 °C), and the minimum, in the Altai Mountains, is −76 °F (−60 °C). Rainfall, which occurs mainly in the summer, averages less than 16 inches (400 mm) per year in the north, 20 to 24 inches (500–600 mm) in the taiga zone, and 12 to 16 inches (300–400 mm) on the steppes. The western slopes of the Altai receive as much as 62 inches (1,575 mm) per year. Snow cover lasts for 240 to 270 days in the north and for 160 to 170 days in the south. It is deepest in the forest zone, where it ranges from 24 to 36 inches (60–90 cm), and in the mountains, where it averages 80 inches (200 cm) per year. It is much shallower on the tundra, ranging from 12 to 20 inches (30–50 cm), and very thin on the steppe, where 8 to 16 inches (20–40 cm) fall.

On the upper Ob the spring floods begin early in April, when the snow on the plains is melting; and they have a second phase, ensuing from the melting of snow on the Altai Mountains. The middle Ob, scarcely affected by the upper Ob's phases, has one continuous spring-summer period of high water, which begins in mid April. For the lower Ob, high water begins in late April or early May. Levels, in fact, begin to rise when the watercourse is still obstructed by ice; and maximum levels, which occur by May on the upper Ob, may not be reached until June, July, or even August on the lower reaches. For the upper Ob, the spring floods end by July, but autumn rains bring high water again in September and October; in the middle and lower Ob, the spring and summer floodwaters gradually recede until freezing sets in. On the lower reaches, flooding may last four months. Flooding of the Ob proper and of the Irtysh obstructs the minor tributaries' drainage.

Ice forms on the Ob from the end of October to the second week of November, after which the lower reaches begin to freeze solid. By the last week of November the entire river is frozen; the upper reaches remain frozen for some 150 days, the lower for 220. The thawing of the ice—which takes longer than the freezing—lasts from the end of April (upstream) to the end of May, and the spring drift (about five days in duration) produces considerable ice jams. The difference in level between high water and low is 25 feet (8 metres) at Novosibirsk on the upper Ob; it reaches 43 feet (13 metres) at Aleksandrovskoye on the middle Ob but decreases to no more than 20 feet (6 metres) at Salekhard near the mouth. The water is warmest in July, reaching a maximum of 82 °F (28 °C) in the vicinity of Barnaul.

The Ob has the third greatest discharge of Siberia's rivers, after the Yenisey and the Lena. On average, it pours some 95 cubic miles (400 cubic km) of water annually into the Arctic Ocean—about 12 percent of that ocean's total intake from drainage.

The volume of flow at Salekhard, just above the delta, is about 1,500,000 cubic feet (42,000 cubic metres) per second at its maximum and 70,000 cubic feet (2,000 cubic metres) per second at its minimum, while for Barnaul, on the upper Ob, the corresponding figures are 340,000 and 5,700 cubic feet (9,600 and 200 cubic metres) per second. The average annual discharge rate at the river's mouth is about 448,500 cubic feet (12,700 cubic metres) per second. Most of the water comes from the melting of seasonal snow and from rainfall; much less of it comes from groundwater, mountain snow, and glaciers.

The waters of the Ob are only slightly mineralized: dissolved substances account for an annual outpouring of 30.2 million tons into the Kara Sea. The average amount of solid matter discharged annually by the Ob totals only about 50 million tons.

Rich meadows extend in bands 1 to 2 miles (2 to 3 km) wide for great distances along the banks of the Ob and cover many of the numerous islands. Pine, cedar, silver fir, aspen, and birch also grow on the banks and occasionally constitute isolated forests on the higher ground of the floodplain. Large areas near the river are covered with willow, snowball trees (Viburnum), bird cherry (Prunus padus), buckthorn (Hippophaë), currant bushes, and wild roses.

Of some 50 species of fish found in the river or in the gulf, the most valuable economically are several varieties of sturgeon and such “whitefish” as nelma (Stenodus leucichthys nelma), muksun (Coregonus muksun), tschirr (C. nasus), and peled (C. pelea); pike, burbot, Siberian dace, carp, and perch are also caught. The seasonal ice cover, however, causes depletion of oxygen in the water, killing many fish every winter in the reaches between the Tym confluence and the delta.

Fur-bearing mammals of the Ob valley include European and Siberian mole, Siberian and American mink, ermine, fox, wolf (in the taiga), elk, white hare, water rat, muskrat, otter, and beaver. Among more than 170 species of birds breeding in the floodplain are grouse, partridge, goose, and duck.

Politically, most of the Ob basin is within Russia, but its southern portion forms the northernmost part of Kazakhstan (Kazakh). Russians and other Slavs (Slav) constitute the majority of the population, but there also are numerous non-Slavic peoples. These include the Kazakhs (Kazakh) in the south, the Altay and Shor peoples of the mountains, the Tatars (Tatar) of the Irtysh basin, the Khanty (Khanty and Mansi) (Ostyak) and the Mansi (Khanty and Mansi) (Vogul)—whose autonomous district ( Khanty-Mansi) occupies part of the taiga—and the Nenets, Nganasan, Enets, and Selkup peoples of the north. The valleys of the river are more densely populated than other parts of the basin.

The Ob, one of western Siberia's principal means of transportation, is navigable for about 190 days of the year on its upper reaches and for 150 on its lower. Both imports and exports are shipped along the river. Most goods are transported to and from it along the northern sea route, which stretches across the Arctic. The Trans-Siberian Railway crosses the Irtysh at Omsk and the upper Ob at Novosibirsk. Railways to Kazakhstan from Novosibirsk and from the foothills of the mountains cross the upper Ob at Barnaul.

The Ob's immense hydroelectric (hydroelectric power) potential is estimated at some 250 billion kilowatts. Three main stations have been built: one on the Ob proper, at Novosibirsk, and the other two on the mountainous reaches of the Irtysh, at Bukhtarma and Öskemen.

Both industry and agriculture have been intensively developed in the Ob basin. Cities such as Omsk, Novosibirsk, and Barnaul are major industrial and manufacturing centres. The steppe zone, in the southern Ob basin, is the major producer of spring wheat in Russia. The west Siberian oil and gas fields, located in the taiga and tundra zones of the middle and lower Ob, are the most important in Russia, contributing about two-thirds of the country's crude oil and natural gas output.