stylistic:tone
Major tributaries

Over 95 significant tributaries and hundreds of smaller ones feed the Missouri River, with most of the larger ones coming in as the river draws close to the mouth. Most rivers and streams in the Missouri River basin flow from west to east, following the incline of the Great Plains; however, some eastern tributaries such as the James, Big Sioux and Grand River systems flow from north to south.

The Missouri's largest tributaries by runoff are the Yellowstone in Montana and Wyoming, the Platte in Wyoming, Colorado, and Nebraska, and the Kansas – Republican / Smoky Hill and Osage in Kansas and Missouri. Each of these tributaries drains an area greater than 50,000 square miles (130,000 km) or has an average discharge greater than 5,000 cu ft/s (140 m /s). The Yellowstone River has the highest discharge, even though the Platte is longer and drains a larger area. In fact, the Yellowstone's flow is about 13,800 cu ft/s (390 m /s) – accounting for sixteen percent of total runoff in the Missouri basin and nearly double that of the Platte. On the other end of the scale is the tiny Roe River in Montana, which at 201 feet (61 m) long is one of the world's shortest rivers.

The table on the right lists the ten longest tributaries of the Missouri, along with their respective catchment areas and flows. Length is measured to the hydrologic source, regardless of naming convention. The main stem of the Kansas River, for example, is 148 miles (238 km) long. However, including the longest headwaters tributaries, the 453-mile (729 km) Republican River and the 156-mile (251 km) Arikaree River, brings the total length to 749 miles (1,205 km). Similar naming issues are encountered with the Platte River, whose longest tributary, the North Platte River, is more than twice as long as its mainstream.

The Missouri's headwaters above Three Forks extend much farther upstream than the main stem. Measured to the farthest source at Brower's Spring, the Jefferson River is 298 miles (480 km) long. Thus measured to its highest headwaters, the Missouri River stretches for 2,639 miles (4,247 km). When combined with the lower Mississippi, the Missouri and its headwaters form part of the fourth-longest river system in the world, at 3,745 miles (6,027 km).