It's a tricky thing, trying to figure out the length of a specific river, let alone determining the longest river in the U.S. To start, you have to decide whether or not the river’s tributaries will count toward the full length. That can be a struggle since it’s not always clear where a tributary begins or ends.
Even the United States Geological Survey (USGS) admits that “somewhat arbitrary decisions” are necessary — alongside maps, aerial photographs and other considerations — when it comes to measuring the lengths of rivers.
That said, when the USGS attempted to compile data on the country's most significant rivers, it did so by considering each candidate’s “total length from source to mouth.” Using that criteria, the agency cataloged the following 10 as the longest rivers in the United States.
1. The Missouri River
People disagree as to whether the Missouri or the Mississippi is the longest river in the U.S., but according to the USGS, the Missouri River comes out ahead by 200 miles (322 km), totaling an impressive 2,540 miles (4,088 km) from source to mouth.
Originating in the Rocky Mountains, the "Big Muddy" river flows through seven states (Montana, North Dakota, South Dakota, Nebraska, Iowa, Kansas and Missouri) before converging with the mighty Mississippi River around St. Louis.
The Missouri River watershed totals more than 525,000 square miles (845,000 square km), providing life-giving water for local plants and animals. The Blackfeet, Crow and other Native American peoples called the upper Missouri River home for centuries. And it was up this river the adventurers Lewis and Clark famously traveled before taking the Columbia River on to the Pacific Ocean.
2. The Mississippi River
Stretching 2,340 miles (3,766 km), the legendary Mississippi is either the country’s longest or second-longest river, depending on who is doing the measuring.
Experts can agree, however, that it’s one of the largest rivers in the world and the country's largest river in terms of its drainage area, which spans 31 states and 1.2 million square miles (3.1 million square km), or roughly one-eighth of North America.
Some lists even categorize the Mississippi river system as one of the longest rivers in the world. Once the boundary between the American West and the rest of the country, the river flows from Minnesota through 10 states before finally emptying into the Gulf of Mexico.
3. The Yukon River
Emerging from the Pelly Mountains in Canada's Yukon Territory in British Columbia, the mighty Yukon River flows 1,980 miles (3,187 km) through the rugged landscapes of Alaska, eventually pouring into the Bering Sea.
Its drainage basin, which is larger than the country of Turkey, covers 328,000 square miles (52,865 square km) throughout the Canadian province and the Alaskan wilderness.
On its way to the Bering Sea, this waterway serves as a primary means of transportation, connecting isolated villages and providing access to essential resources like salmon and fresh water.
4. The Rio Grande
Meandering 1,900 miles (3,058 km), the Rio Grande (or “Big River” in Spanish) forms a natural border between the U.S. and Mexico. Originating in the San Juan Mountains of Colorado, the river flows through New Mexico and Texas before reaching the Gulf of Mexico, for a total watershed of 336,000 square miles (540,740 square km).
The Rio Grande river has carved several impressive canyons along its course, including the Santa Elena Canyon in Texas, which is known for its towering limestone cliffs and stunning views. However, its unpredictable flow has put it at the center of numerous disputes between the U.S. and Mexico over water rights and allocation of water for all the agricultural land and municipalities.
5. The Arkansas River
The headwaters of the Arkansas originate in the Rocky Mountains of Colorado, near the Continental Divide, and journey 1,469 miles (2,364 km) through Kansas and Oklahoma before joining the Mississippi River in Arkansas.
Renowned as one of the top whitewater rafting destinations in the country, the Arkansas River attracts adventure seekers from near and far who want to experience its thrilling rapids, including the famous Royal Gorge section. Its watershed is 161,000 square miles (259,104 square km).
6. The Colorado River
The iconic Colorado River system, which covers a distance of 1,450 miles (2,334 km), has two major claims to fame. First, it carved the awe-inspiring Grand Canyon over millions of years. Second, it’s dammed at several points, the most well-known being the Hoover Dam.
With a watershed of 246,000 square miles (637,000 square km), the Colorado serves as a crucial source of water and hydropower power for urban areas like Los Angeles, Las Vegas and Phoenix. Its hydroelectric power supports more than 40 million people in seven U.S. states and Mexico.
Beginning in the Rocky Mountains, the Colorado River flows west and south until it reaches the Gulf of California and the Pacific Ocean. In fact, it has the distinction of being the only river in this list that flows toward the Pacific Ocean.
7. The Atchafalaya River
At 1,420 miles (2,285 km) long, the Atchafalaya River plays a critical role in safeguarding the unique ecosystems of Louisiana. Serving as a distributary of the Mississippi, it splits from the main channel at a point near Simmesport, Louisiana, and flows to the Gulf of Mexico, essentially acting as a natural flood relief valve.
This, in addition to flood control solutions by the U.S. Army Corps of Engineers, helps to mitigate the risk of catastrophic flooding in the downstream areas. With a watershed of 95,100 square miles (246,300 square km), the river’s basin is the largest floodplain swamp in North America.
8. The Ohio River
Formed by the confluence of the Allegheny and Monongahela Rivers in Pittsburgh, Pennsylvania, the Ohio River flows for 1,310 miles (2,108 km), serving as the largest tributary of the Mississippi River. Its watershed is 203,000 square miles (326,697 square km).
Numerous other rivers feed the Ohio, including the Tennessee River, Cumberland River and Kanawha River. In addition to Pittsburgh, other major American cities developed along its banks, including Cincinnati, Ohio, and Louisville, Kentucky.
9. The Red River
Originating in New Mexico, the Red River flows through the Great Plains, including through Texas, Oklahoma, Arkansas and Louisiana, before merging with the Mississippi River after a total of 1,290 miles (2,076 km). For roughly 640 of those miles (1,030 km), the river actually serves as the boundary between Texas and Oklahoma.
Researchers estimate the Red River’s drainage system to be 93,200 square miles (149,991 square km), although the many dams and reservoirs along its tributaries mean the river’s flow across the Great Plains can vary widely at different times and according to different sources.
10. The Brazos River
Meandering for 1,280 miles (2,060 km), the Brazos is the tenth longest river in the U.S. but the longest entirely within the state of Texas. The river flows from the panhandle through the heart of the Lone Star state to the Gulf of Mexico.
It just barely beats the length of another major North American river, the Columbia River, which flows for 1,240 miles (1,996 km) from the Canadian province of British Columbia through the Pacific Northwest toward the Pacific Ocean.
Dammed at multiple points to create reservoirs, including Lake Possum Kingdom and Lake Whitney, the Brazos River has a watershed of 45,600 square miles (7,339 square km). The river’s original name was Brazos de Dios, or “Arms of God” in Spanish.
This article was created in conjunction with AI technology, then was fact-checked and edited by a HowStuffWorks editor.
Original article: 10 Longest Rivers in the U.S.: From the Missouri to the Brazos
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