Just as the nation's most famous river is undergoing historic and potentially catastrophic flooding, the environmental group American Rivers has come out with its annual top 10 list of the USA's "most endangered" rivers.

The most endangered river in the USA, according to the group, is the Susquehanna River, due primarily to natural gas development and the hazards associated with hydraulic fracturing, also known as "fracking."

The Susquehanna, which flows from New York State and through Pennsylvania and Maryland before emptying into the Chesapeake Bay, provides drinking water for more than six million people in the Northeast and mid-Atlantic.

"Fracking poses one of the greatest risks our nation's rivers have faced in decades," says Andrew Fahlund, senior vice president of conservation at American Rivers. "We are taking a major gamble on the clean drinking water for millions of Americans."

Fracking was also the reason for the listing of the seventh most endangered river, the Hoback River in western Wyoming.

Fracking is a form of drilling for natural gas from shale rock. The process blasts waters, mixed with sand and chemicals, deep underground to fracture shale formation and allow natural gas to escape. Some of the liquid then comes back to the surface.

An industry spokesman disagrees that fracking is a problem: "These guys are in way over their head here," says Chris Tucker, a spokesman for Energy In Depth, a group that supports USA's natural gas and oil industry. "Somewhere along the way, I'm afraid they were sold a bill of goods about what hydraulic fracturing is, and how it's actually used in a modern context.

"As someone who grew up on the banks of the Susquehanna, that river certainly has its fair share of problems," Tucker adds. "The regulated use of hydraulic fracturing technology in rock strata two miles beneath the riverbed is not one of them. And it's a complete farce to suggest that it is."

Overall, the 3.5 million miles of rivers and streams in the USA provide 65 percent of the nation's drinking water, says American Rivers spokeswoman Amy Kober. "Nearly everyone in the USA lives within a mile of a river," she says.

Here is the complete top 10 list:

Susquehanna River (New York, Pennsylvania, Maryland) Bristol Bay (Alaska) Roanoke River (Virginia, North Carolina) Chicago River (Illinois) Yuba River (California) Green River (Washington) Hoback River (Wyoming) Black Warrior River (Alabama) St. Croix River (Minnesota, Wisconsin) Ozark National Scenic Riverways (Missouri)

Besides fracking, other reasons for these rivers making the list include mining, sewage pollution, development, overuse and poor planning.

According to American Rivers, the report is not a list of the "worst" or most polluted rivers in the country. Rivers that are endangered could actually be in pristine condition or severely degraded. The key factor, the conservation group says, is that each of the endangered rivers is at a tipping point - they are listed because their fate is likely to be decided in the coming year.

"The efforts of American Rivers to call attention to endangered rivers raises critical public awareness of the need to protect these waters and of the threats they face from pollution and development." says Environmental Protection Agency Acting Assistant Administrator for Water Nancy Stoner.

"Unfortunately, over the past decade, interpretations of Supreme Court rulings have removed some critical waters from federal protection, and caused confusion about which waters and wetlands are protected from pollution and development," Stoner adds. "EPA recently released draft guidance that clarifies where the Clean Water Act applies nationwide."

American Rivers gave a "special mention" to the Mississippi this year, pointing to outdated flood management strategies and over-reliance on levees that have contributed to the record flood damage. Levees should be the last line of defense, not our only line of defense, according to the report.

The Mississippi has been on the most endangered rivers list eight times, with threats ranging from flood control to pollution.