Operation - or mis-operation - of Glen Canyon Dam has turned the Colorado River below it into America's "most endangered" river, according to the Washington-based environmental group American Rivers.

And Utah's Fremont River near Capitol Reef National Park is also on the group's list of "highly threatened" rivers because of a proposed dam and hydroelectric proj-ect it says may dry up the river."Our hope is that this announcement of the endangered rivers not only helps to protect these specific rivers, but raises river conservation to the top of our nation's environmental agenda," said Kevin Coyle, president of American Rivers.

The group placed the stretch of the Colorado River below Glen Canyon Dam at the top of its annual list of the 10 most endangered rivers because of damage from wildly fluctuating water releases from the dam to enhance hydroelectric power production.

American Rivers statements said such releases "can cause the level of the river to rise and fall 13 feet in a single day."

It says that (and preventing natural migration of sediment by the dam) destroy fragile beaches downstream throughout the Grand Canyon, and harm vegetation, archaeological sites, river running, a prized trout fishery and habitat for many species including the federally endangered humpback chub.

American Rivers put the Colorado on the top of its list to support legislation pushed by Rep. George Miller, D-Calif., and Sen. John McCain, R-Ariz., that would direct the U.S. Bureau of Reclamation to operate Glen Canyon Dam in such a way to protect and enhance downstream environmental, recreational and cultural resources.

The Interior Department is currently conducting studies about the effect that varying river flows has on the Grand Canyon.

Rep. Jim Hansen, R-Utah, ranking Republican on the House Interior Subcommittee on Water, Power and Offshore Resources is among members of Congress who support reducing the extremes in flow fluctuations, said James Barker, a Hansen aide on the committee.

Rivers were listed because of threats from dams, mines, pollution and land development.

The environmental group also released a list of 15 "high threatened" rivers, which included Utah's Fremont in the Fremont Gorge near Capitol Reef National Park.

It said the Fremont is endangered by a 110-foot high dam and hydroelectric plant proposed west of Torrey by the Wayne County Water Conservancy District. Backers in Wayne County say the project would double farm land in the region and makemoney from power sales as well.

The group said the dam would destroy "a fast-moving desert river in a deep gorge (with) numerous side canyons, excellent fishing, hiking, seasonal waterfalls and solitude."

It added it feels the proposed dam "would essentially dewater the river in the Fremont Gorge unit and drastically affect vegetation in Capitol Reef."

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Endangered rivers

The 10 U.S. rivers most endangered by hydroelectric dams, mining operations, pollution, land development and other man-made threats:

1. Colorado River, Arizona

2. Alsek and Tatshenshini rivers, Alaska and Canada

3. American River, California

4. Penobscot River, Maine

5. Susquehanna River, Pennsylvania

6. Upper Mississippi River, Upper Midwest

7. Columbia and Snake rivers, Northwest

8. Gunnison River, Colorado

9. Passaic River, New Jersey

10. New River, North Carolina