ouri's drainage basin has highly variable weather and rainfall patterns, Overall, the watershed is defined by a Continental climate with warm, wet summers and harsh, cold winters. Most of the watershed receives an average of 8 to 10 inches (200 to 250 mm) of precipitation each year. However, the western most portions of the basin in the Rockies as well as southeastern regions in Missouri may receive as much as 40 inches (1,000 mm). The vast majority of precipitation occurs in summer in most of the lower and middle basin, although the upper basin is known for short-lived but intense summer thunderstorms such as the one which produced the 1972 Black Hills flood through Rapid City, South Dakota. Winter temperatures in the northern and western portions of the basin typically drop to -20 °F (-29 °C) or lower every winter with extremes as low as −60 °F (−51 °C), while summer highs occasionally exceed 100 °F (38 &de g;C) in all areas except the higher elevations of Montana, Wyoming and Colorado. Extreme maximums have exceeded 115 °F (46 °C) in all the states and provinces in the basin - almost all prior to 1960. As one of the continent's most significant river systems, the Missouri's drainage basin borders on many other major watersheds of the United States and Canada. The Continental Divide, running along the spine of the Rocky Mountains, forms most of the western border of the Missouri watershed. The Clark Fork and Snake River, both part of the Columbia River basin, drain the area west of the Rockies in Montana, Idaho and western Wyoming. The Columbia, Missouri and Colorado River watersheds meet at Three Waters Mountain in Wyoming's Wind River Range. South of there, the Missouri basin is bordered on the west by the drainage of the Green River, a tributary of the Colorado, then on the south by the mainstem of the Colorado. Both the Colorado and Columbia Rivers flow to the P acific Ocean. However, a large endorheic drainage called the Great Divide Basin exists between the Missouri and Green watersheds in western Wyoming. This area is sometimes counted as part of the Missouri River watershed, even though its waters do not flow to either side of the Continental Divide. To the north, the much lower Laurentian Divide separates the Missouri River watershed from those of the Oldman River, a tributary of the South Saskatchewan River, as well as the Souris, Sheyenne, and smaller tributaries of the Red River of the North. All of these streams are part of Canada's Nelson River drainage basin, which empties into Hudson Bay. There are also several large endorheic basins between the Missouri and Nelson watersheds in southern Alberta and Saskatchewan. The Minnesota and Des Moines Rivers, tributaries of the upper Mississippi, drain most of the area bordering the eastern side of the Missouri River basin. Finally, on the south, the Ozark Mountains and other low divi des through central Missouri, Kansas and Colorado separate the Missouri watershed from those of the White River and Arkansas River, also tributaries of the Mississippi River. Major tributaries A river flows past grass-covered banks, trees are in the midground The Yellowstone River, the fifth longest tributary of the Missouri, which it joins in North Dakota Main article: List of tributaries of the Missouri River 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 km2) or has an average discharge greater than 5,000 cu ft/s (140 m3/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 m3/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. Longest tributaries of the Missouri River 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). Discharge Aerial view of farms and a power station in a rural area partly inundated by a river that has overflowed its banks Nebraska's Fort Calhoun Nuclear Generating Station was inundated when the Missouri River flooded in 2011 By discharge, the Missouri is the ninth largest river of the United States, after the Mississippi, St. Lawrence, Ohio, Columbia, Niagara, Yukon, Detroit, and St. Clair. The latter two, however, are sometimes considered part of a strait between Lake Huron and Lake Erie. Among rivers of North America as a whole, the Missouri is thirteenth largest, after the Mississippi, Mackenzie, St. Lawrence, Ohio, Columbia, Niagara, Yukon, Detroit, St. Clair, Fraser, Slave, and Koksoak. As the Missouri drains a predominantly semi-arid region, its discharge is much lower and more variable than other North Amer