· Missouri: 2,540 miles
· Mississippi: 2,340 miles
· Yukon: 1,980 miles
· Rio Grande: 1,900 miles
· St. Lawrence: 1,890 miles
The Missouri River is the longest river in North America and a major waterway of the central United States. Rising in the Rocky Mountains of western Montana, the Missouri flows east and south for 2,341 miles (3,767 km) before entering the Mississippi River north of St. Louis, Missouri. The river takes drainage from a sparsely populated, semi-arid watershed of more than half a million square miles (1,300,000 km2), which includes parts of ten U.S. states and two Canadian provinces.
For over 12,000 years, people have depended on the Missouri and its tributaries as a source of sustenance and transportation. More than ten major groups of Native Americans populated the watershed, most leading a nomadic lifestyle and dependent on the enormous buffalo herds that once roamed through the Great Plains. The first Europeans discovered the river in the late seventeenth century, and the region passed through Spanish and French hands several times before finally becoming part of the United States through the Louisiana Purchase. The Missouri was long believed to be part of the Northwest Passage – a water route from the Atlantic to the Pacific – but when Lewis and Clark became the first to travel the river's entire length, they confirmed the nonexistence of the mythical pathway.
The Missouri was one of the main routes for the westward expansion of the United States during the 19th century. The growth of the fur trade in the early 1800s laid much of the groundwork as trappers explored the region and blazed trails. Pioneers headed west en masse beginning in the 1830s, first by covered wagon, then by the growing numbers of steamboats entering service on the river. Former Native American lands in the watershed were taken over by settlers, leading to some of the most longstanding and violent wars against indigenous peoples in American history.
During the 20th century, the Missouri River basin was extensively developed for irrigation, flood control and the generation of hydroelectric power. Fifteen dams impound the main stem of the river, with hundreds more on tributaries. Meanders have been cut and the river channelized to improve navigation, reducing its length by almost 200 miles (320 km) from pre-development times. Although the lower Missouri valley is now a populous and highly productive agricultural and industrial region, heavy development has taken its toll on wildlife and fish populations as well as water quality.
The Mississippi River is the largest river system in North America. Flowing entirely in the United States, it rises in western Minnesota and meanders slowly southwards for 2,530 miles (4,070 km) to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains all or parts of 31 U.S. states between the Rocky and Appalachian Mountains and even reaches into southern Canada. The Mississippi ranks fourth longest and tenth largest among the world's rivers.
Native Americans lived along the Mississippi and its tributaries. Most were hunter-gatherers or herders, but some, such as the Mound builders, formed prolific agricultural societies. The arrival of Europeans in the 1500s forever changed the native way of life as first explorers, then settlers, ventured into the basin in increasing numbers. The river served first as barrier – forming borders for New Spain, New France, and the early United States – then as vital transportation artery and communications link. In the 19th century, during the height of Manifest Destiny, the Mississippi and several western tributaries, most notably the Missouri, formed pathways for pioneers partaking in the western expansion of the United States.
Formed from thick layers of this river's silt deposits, the Mississippi River Valley is one of the most fertile agricultural regions of the country, which resulted in the river's storied steamboat era. During the American Civil War, the Mississippi's capture by Union forces marked a turning point towards victory because of this very importance as a route of trade and travel, not least to the Confederacy. Because of substantial growth of cities and the larger ships and barges that have supplanted riverboats, the decades following the 1900s saw massive engineering works applied to the river system, such as the often in-combination construction of levees, locks and dams.
Since modern development of the basin began, the Mississippi has also seen its share of pollution and environmental problems – most notably large volumes of agricultural runoff, which has led to the Gulf of Mexico dead zone off the Delta. In recent years, the river has shown a steady shift towards the Atchafalaya River channel in the Delta; a course change would prove disastrous to seaports such as New Orleans. A system of dikes and gates has, so far, held the Mississippi at bay but, due to fluvial processes, the shift becomes more likely each year.
The Yukon River is a major watercourse of northwestern North America. The source of the river is located in British Columbia, Canada. The next portion lies in, and gives its name to Yukon Territory. The lower half of the river lies in the U.S. state of Alaska. The river is 1,980 miles (3,190 km) long and empties into the Bering Sea at the Yukon-Kuskokwim Delta. The average flow is 6,430 m³/s (227,000 ft³/s). The total drainage area is 832,700 km² (321,500 mi²), of which 323,800 km² (126,300 mi²) is in Canada. By comparison, the total area is more than 25% larger than Texas or Alberta.
The longest river in Alaska and the Yukon Territory, it was one of the principal means of transportation during the 1896–1903 Klondike Gold Rush. A portion of the river in the Yukon—"The Thirty Mile" section, from Lake Laberge to the Teslin River—is a national heritage river and a unit of Klondike Gold Rush International Historical Park. Paddle-wheel riverboats continued to ply the river until the 1950s, when the Klondike Highway was completed. After the purchase of Alaska by the United States in 1867, the Alaska Commercial Company acquired the assets of the Russian-American Company and constructed several posts at various locations on the Yukon River.
Yukon means "great river" in Gwich'in. The river was called Kuigpak, or "big river", in Central Yup'ik. The Lewes River is the former name of the upper course of the Yukon, from Marsh Lake to the confluence of the Pelly River at Fort Selkirk.
The Yukon River has had a history of pollution from gold mining, military installations, dumps, wastewater, and other sources. However, the Environmental Protection Agency does not list the Yukon River among its impaired watersheds, and water quality data from the U.S. Geological Survey shows relatively good levels of turbidity, metals, and dissolved oxygen.
The Yukon River Inter-Tribal Watershed Council, a cooperative effort of 64 First Nations and tribes in Alaska and Canada, has the goal of making the river and its tributaries safe to drink from again by supplementing and scrutinizing Government data.
