American Rivers, a conservation group that compiles the annual list, chose the Sacramento and the San Joaquin rivers because their collapse could endanger the water supply of 25 million Californians, flood the state's capital and damage the delicate freshwater delta where the two rivers twine.
"The health of the delta depends directly on maintaining the health of these two rivers that feed it," said Steve Rothert, California director of the Washington-based nonprofit.
The organization chooses its most endangered rivers from nominations made by environmental groups and considers the value of each river to people and the environment, the level of the threat it faces and pending decisions that could affect it in the next year, Rothert said.
But Jerry Johns, deputy director at the Department of Water Resources, said the report did not give adequate consideration to statewide efforts to restore the health of the delta and its two chief tributaries.
Rivers from Pennsylvania to Alaska also made this year's list. Rounding out the top five were Georgia's Flint River, the Lower Snake River that courses through Idaho, Washington and Oregon, Mattawoman Creek in Maryland and the North Fork of the Flathead River in Montana.
One recent political compromise will help restore a now-dry 60-mile stretch of the San Joaquin River.
Last week,that implements a 2006 legal settlement to bring water and Chinook salmon back to a portion of the state's second-longest river. It provides about $390 million in federal and state funds, as well as fees from water users, over the next decade.
The lawsuit stemmed from the opening of Friant Dam in 1949, which transformed the San Joaquin Valley's main artery from a river thick with salmon into an irrigation source for more than a million acres of farm fields.
Under the 2006 settlement, irrigation districts that distribute river water to thousands of farms agreed to relinquish a set portion of their traditional water supplies to help restore the fish.
- Sacramento-San Joaquin River Delta System, Calif. (Threat: Outdated water supply and flood management systems)
- Flint River, Ga. (Threat: Proposed water supply dams)
- Lower Snake River, Id., Ore., Wash. (Threat: Four dams)
- Mattawoman Creek, Md. (Threat: Proposed highway in Charles County)
- North Fork of the Flathead River, Mt. (Threat: Proposed mines)
- Saluda River, S.C. (Threat: Sewage pollution)
- Laurel Hill Creek, Pa. (Threat: Water withdrawals)
- Beaver Creek, Ak. (Threat: Proposed oil and gas development)
- Pascagoula River, Miss. (Threat: New petroleum storage)
- Lower St. Croix National Scenic Riverway, Minn., Wisc. (Threat: Loss of "Wild and Scenic River" protections)