The Missouri River is the longest river in North America, beating the Mississippi by 200 miles, according to the U.S. Geological Survey. The Missouri begins in the Rocky Mountains of Montana, where three smaller rivers—the Jefferson, Madison and Gallatin Rivers—converge to form the headwaters of the Missouri. The river ends 15 miles north of St. Louis, Missouri, where it spills into the Mississippi.

Since the Missouri is a tributary of the Mississippi, they’re essentially one continuous body of water. Taken together, the combined Missouri-Mississippi would be the fourth-longest river in the world at 3,710 miles.

The Missouri is known as “The Big Muddy” because of the massive amount of soil and silt that the river carries as it crosses the Great Plains. As part of American history, the Missouri is best known from the 1805 and 1806 expeditions of Meriwether Lewis and William Clark. Lewis and Clark—along with Sacagawea, their Indigenous interpreter and guide—trekked nearly the entire length of the Missouri as they charted the newly acquired Louisiana Purchase.

The Missouri River watershed is one of the largest in North America—more than 500,000 square miles—and captures water from 10 states and two Canadian provinces. A vital water source, the Missouri and its tributaries irrigate a quarter of all agricultural land in the United States.

More than 150 species of fish and 300 species of birds are native to the Missouri River basin. The upper Missouri is the last stronghold for the endangered pallid sturgeon, a prehistoric freshwater fish that can weigh up to 85 pounds and live for 100 years.

After a devastating flood in 1943, the U.S. Army Corps of Engineers launched an ambitious project to redevelop the Missouri River, constructing hydroelectric dams, reservoirs, levees and irrigation canals. Although the project was a boon for agriculture and flood control, it altered the flow of water in ways that destroyed many natural habitats and wetlands.

The Mississippi River may be shorter than the Missouri, but no river has played a larger role in American commerce and culture than the “Mighty Mississippi.” Mark Twain set his iconic American novel The Adventures of Tom Sawyer along the western bank of the Mississippi in Missouri, and the Mississippi Delta was the birthplace of the Blues.

On a map, the Mississippi River runs almost perfectly north to south. The source of the Mississippi is a quiet mountain stream just 20-feet wide that spills from Lake Itasca in Minnesota. Over its length, the Mississippi passes through 10 states—Minnesota, Wisconsin, Iowa, Illinois, Missouri, Kentucky Arkansas, Tennessee, Louisiana and Mississippi—and grows as wide as half a mile—large enough to accommodate large commercial vessels and barges.

“The Mississippi is such an important transportation corridor, because it runs in more or less a straight line from Minnesota, a northern state, all the way down to the Gulf of Mexico,” says Taylor. “It effectively brings the sea inland for thousands of miles and enables very efficient transportation and trade. It’s really a vital waterway.”

The Mississippi River is an equally important corridor for wildlife, with almost half of all American waterfowl species migrating along the Mississippi and its tributaries.

“It’s part of a major flyway for birds transitioning from southern overwintering areas to northern breeding grounds,” says Taylor. “They depend on the Mississippi and its associated wetlands to sustain them.”

Like the Missouri, the flow of the Mississippi River was significantly altered by 20th-century engineering projects to straighten and deepen the river for better commercial access. Those human interventions—which effectively cut off the river from its natural floodplain—have resulted in flooding and habitat loss. The nonprofit American Rivers has named the Mississippi the most endangered river in America.

The third-longest river in the United States is shared with Canada. The nearly 2,000-mile Yukon River begins at the border between British Columbia and the Yukon Territory, then crosses northwestward into Alaska on a long curving arc to the Bering Sea.

In the Indigenous Gwich'in language, Yukon means “great river,” and archeologists believe that the Yukon River may have been home to some of the first human settlements in North America.

“When the first Native North Americans crossed the Bering land bridge, the lower Yukon River was important as an early settlement site,” says Taylor. “Before the ice sheets receded, this region was thought to be a staging area for later colonization routes to the south.”

In 1896, news spread of a historic gold strike near Dawson City in the Yukon Territory. The Yukon River and its tributaries drew thousands of miners desperate to get rich from the Klondike Gold Rush and countless businesses eager to exploit them.

The Yukon River is one of the most important salmon-breeding rivers in the world. It’s home to three native salmon species: chum, coho and Chinook. Each year, Chinook salmon migrate more than 1,500 miles upstream from the ocean to their freshwater breeding grounds at the headwaters of the Yukon River. One of their final challenges is to scale the 1,200-foot wooden fish ladder at the Whitehorse Dam, the largest in the world.

Taylor says that climate change and other factors have severely impacted the health of the salmon populations in the Yukon River.

“The salmon runs have done very poorly over the last 10 to 15 years,” says Taylor. “The salmon in the Yukon Basin are so important, not only for commercial fisheries, but also for food security for Indigenous people, and really for the health of the entire ecosystem.”

The story of the Rio Grande River begins over 12,000 feet up in the San Juan Mountains of southern Colorado. There, trickles of water from the melting snowpack drain into clear mountain streams that cascade down through pine-forested slopes. The Rio Grande becomes a proper river in the verdant San Luis Valley of southern Colorado, an important stopover point for hundreds of species of migrating birds, including the iconic sandhill crane.

Flowing nearly 2,000 miles from Colorado to the Gulf of Mexico, the Rio Grande has always been the lifeblood of the American Southwest. From the ancient Clovis people that first populated the region 12,000 years ago, through Spanish conquest and Western expansion, the Rio Grande has been the chief water source for an otherwise parched landscape.

“The Rio Grande is basically flowing through a desert,” says Taylor, “and it's hugely important in terms of supplying drinking water to six million people. It's a vital artery for human persistence and the persistence of all sorts of animal and plant life in that area.”

Despite its immense value and importance to the Southwest ecosystem, the Rio Grande has been heavily degraded. Threats include climate change and pollution but mostly overallocation—there simply isn’t enough water to meet the increasing demands of agriculture, industry and residential development.

In most years, the flow of the mighty Rio Grande is reduced to a thin trickle, sometimes too weak to connect with the Gulf of Mexico. The fate of the Rio Grande, one of America’s longest and most storied rivers, is very much an open question. American Rivers lists the Lower Rio Grande as the fifth most-endangered river in America.

Americans might be surprised to see the St. Lawrence River tied with the Rio Grande as one of the five longest rivers in the United States. But according to the U.S. Geological Survey, the St. Lawrence River is much longer than the 744-mile “main stem” that flows out of Lake Ontario past Montreal and Quebec, emptying into the Gulf of St. Lawrence.

Instead, the USGS groups the St. Lawrence together with the five Great Lakes, the largest freshwater system in the world, bringing the total length of the St. Lawrence River-Great Lakes system to 1,900 miles. There’s a logic to this, because the Great Lakes flow into each other—from west to east—and then empty into the St. Lawrence River, which takes all of that freshwater to sea.

“You could argue that any one of the Great Lakes could be the source of the St. Lawrence,” says Taylor. “Together, the Great Lakes basin holds 20 percent of the world’s freshwater. The other end, where the St. Lawrence meets the sea, is the largest estuary in the world. The St. Lawrence is a very special river.”

The St. Lawrence River-Great Lakes system has nurtured wildlife and human settlement for thousands of years. When the first Europeans arrived in the 16th century, they estimated that as many as 117,000 Native Americans lived around the Great Lakes. Today, the 1,900-mile system is home to more than 45 million people. The lakes, the river and the St. Lawrence estuary are home to countless species of birds, fish and land animals.

Together, the Great Lakes and St. Lawrence River form a large section of the natural border between the United States and Canada, so the waterways are managed by binational agreements. In the 1950s, the two countries completed one of the great engineering feats of the 20th century, the construction of the St. Lawrence Seaway. More than 22,000 workers moved more than 210 million cubic yards of earth and rock to dig a series of canals and locks that enabled the passage of large seagoing vessels all the way into the Great Lakes.

“The St. Lawrence Seaway was extolled as the largest construction project on Earth at the time, and it created huge opportunities for both countries,” says Taylor.

Unfortunately, with the arrival of international trade came invasive species, none more damaging than the sea lamprey. The suction-mouthed predator spread like a plague throughout the Great Lakes, feeding on native trout, whitefish, perch and sturgeon. Thankfully, 90 percent of the parasitic fish have been eradicated through a binational program—the largest vertebrate-control effort in the world.