<p><em>This gallery is part of a special <a href="http://news.nationalgeographic.com/news/clean_water_crisis.html">National Geographic News series</a> on global water issues.</em></p><p class="MsoNormal">The Susquehanna River drains 27,500 square miles (71,224 square kilometers)<strong> </strong> of land in the eastern two-thirds of Pennsylvania, and parts of bordering states, flowing through the Marcellus Shale natural gas extraction area before emptying into the Chesapeake Bay.</p><p class="MsoNormal">It provides drinking water for more than six million people.</p><p class="MsoNormal">Now in its 26th year, the <a href="http://www.americanrivers.org/">American Rivers</a> Most Endangered Rivers List includes not necessarily the nation's most polluted rivers, but those "at a crossroads, whose fates will be determined in the coming year." The Susquehanna tops out the American Rivers list this year because it's in the Marcellus Shale region, which has become a target in the rush to develop natural gas reserves.</p><p class="MsoNormal">(Read National Geographic News' special report on <a href="http://news.nationalgeographic.com/news/energy/2010/10/101022-energy-marcellus-shale-gas-rush/">the Great Shale Gas Rush</a>.)</p><p class="MsoNormal">The process used to extract the natural gas, called hydraulic fracturing, or "fracking," starts with large quantities of water. The water is mixed with sand and chemicals and pumped underground to create extreme pressure, which cracks non-porous rock formations and allows the natural gas or oil trapped inside to rise to the surface.</p><p class="MsoNormal">Environmentalists worry because most of today's water-treatment facilities can’t adequately treat the toxic—and potentially carcinogenic—wastewater that is generated.</p><p class="MsoNormal">They are concerned that the current rules are inadequate to prevent the contamination of underground and surface drinking water supplies. Accidental spills have already threatened the Susquehanna and its tributaries.</p><p class="MsoNormal">Last year's top endangered river was the Upper Delaware, also threatened by natural gas extraction. Since last year's listing, the Delaware River Basin Commission has begun developing regulations that would be stronger than others in the region.</p><p class="MsoNormal"><em>—Anne Minard</em></p>

Susquehanna River, New York, Pennsylvania, and Maryland

This gallery is part of a special National Geographic News series on global water issues.

The Susquehanna River drains 27,500 square miles (71,224 square kilometers) of land in the eastern two-thirds of Pennsylvania, and parts of bordering states, flowing through the Marcellus Shale natural gas extraction area before emptying into the Chesapeake Bay.

It provides drinking water for more than six million people.

Now in its 26th year, the American Rivers Most Endangered Rivers List includes not necessarily the nation's most polluted rivers, but those "at a crossroads, whose fates will be determined in the coming year." The Susquehanna tops out the American Rivers list this year because it's in the Marcellus Shale region, which has become a target in the rush to develop natural gas reserves.

(Read National Geographic News' special report on the Great Shale Gas Rush.)

The process used to extract the natural gas, called hydraulic fracturing, or "fracking," starts with large quantities of water. The water is mixed with sand and chemicals and pumped underground to create extreme pressure, which cracks non-porous rock formations and allows the natural gas or oil trapped inside to rise to the surface.

Environmentalists worry because most of today's water-treatment facilities can’t adequately treat the toxic—and potentially carcinogenic—wastewater that is generated.

They are concerned that the current rules are inadequate to prevent the contamination of underground and surface drinking water supplies. Accidental spills have already threatened the Susquehanna and its tributaries.

Last year's top endangered river was the Upper Delaware, also threatened by natural gas extraction. Since last year's listing, the Delaware River Basin Commission has begun developing regulations that would be stronger than others in the region.

—Anne Minard

Photograph by Jimmy May, Bloomsburg Press Enterprise/AP

Pictures: America’s Top 10 Endangered Rivers

Environmentalists float their concerns about natural gas development, proposed mines, dams, sewage pollution, and the Mississippi River.

Read This Next

What drives elephant poaching? It’s not greed
How old are you, really? The answer is written on your face.
The rise of vegan safaris

Go Further

Subscriber Exclusive Content

Why are people so dang obsessed with Mars?

How viruses shape our world

The era of greyhound racing in the U.S. is coming to an end

See how people have imagined life on Mars through history

See how NASA’s new Mars rover will explore the red planet

Why are people so dang obsessed with Mars?

How viruses shape our world

The era of greyhound racing in the U.S. is coming to an end

See how people have imagined life on Mars through history

See how NASA’s new Mars rover will explore the red planet

Why are people so dang obsessed with Mars?

How viruses shape our world

The era of greyhound racing in the U.S. is coming to an end

See how people have imagined life on Mars through history

See how NASA’s new Mars rover will explore the red planet