By Conor Mihell

At a time when it's difficult to name a large river that remains free-flowing anywhere in the world, it is refreshing to learn that at least one of the planet's longest waterways remains virtually unknown and relatively pristine. The Amur River flows over 2,500 miles from its source in Mongolia, along the border of Russia and China to the Sea of Okhotsk. This summer, four women will attempt to paddle the Amur as a part of a National Geographic Young Explorers Grant-sponsored initiative to collect scientific data and raise awareness of the challenges facing "global rivers."

Americans Becca Dennis, Sabra Purdy and Amber Valenti, along with Australian photographer Krystle Wright, call their 70-day expedition Nobody's River. That's because political tension on the Russia-China border has kept the Amur free of dams—and also in a perpetual state of limbo. "It's been a benefit to the river, but also a challenge when it comes to its management," says Valenti, a physician assistant who teaches wilderness medicine based in Colorado. "A lot of the major [conservation] players have run into a lot of problems because they're working with two of the most politically difficult countries to work in simultaneously."

Valenti stumbled upon the Amur while daydreaming as a grad school student. As a paddler, she loved the idea of "running a free-flowing river from source to sea." The unlikely Amur River caught her eye for its "amazing history, culture and biodiversity," says Valenti. "It has Siberian tigers and brown bear and passes through seven different ecosystems. That just blew me away.

"Most intriguing was that most American paddlers don't know anything about this river," she adds. "Unlike the Amazon or the Nile or the Congo, it's not on anyone's radar. We want to put the Amur River in the public consciousness. Along with the Amazon, Siberia is one of the world's last strongholds of free-flowing rivers."

Valenti's friend Dennis, a Grand Canyon guide and nursing student in Tucson, Ariz., was an easy recruit. Another friend, Purdy was brought on for her experience as a water ecologist. Wright was enlisted on the advice of a mutual friend. Besides the National Geographic support, the team also won the prestigious Polartec Challenge Grant.

Nobody's River gets underway this month, when the team will travel to Ulaanbaatar, Mongolia, and access the source of the Amur at the Onon River by packhorse. Then they'll assemble TRAK folding sea kayaks and paddle as much of the river as possible, bypassing a few particularly tense border regions by train and bus. Purdy will handle the team's scientific duties, recording acidity, nutrient content, turbidity and other characteristics of the Amur and its tributaries—data that's scarcely known in the watershed.

The river itself is primarily flatwater, so the four women—all whitewater kayakers—will be exchanging the adrenaline of big drops for the thrill of the unknown. "I feel really lucky to travel on a river like this," says Dennis. "There's no guidebook. It's just going to be us exploring it. Not really knowing what sort of experience we're going to have is exhilarating and a little bit overwhelming at the same time."