UTAH

Steep river canyons lined with cottonwood trees are the signature landscape in Utah's Zion National Park. But a new report issued jointly by the park and the Grand Canyon Trust finds that without intervention, the giant trees will likely vanish in the next few decades.

That's because the trees in the lush forests that border the park's Virgin River are all over 70 years old. "There're no youngsters," says Jim McMahon, a former Grand Canyon Trust staffer, who oversaw the study. "This is a retirement community; it's not a healthy, reproducing community."

The reason, according to McMahon, is a century of human manipulation. In the 1930s, the federal government constructed two miles of stone levees to protect roads and park facilities from floods. But the levees have also kept the river from its historic floodplain, where it once deposited sediments vital to cottonwood seed germination.

The report recommends that the Park Service remove the levees - at an estimated cost of $5 million. Park officials say they won't make a decision about restoration for at least another year. First, they'll talk with downstream water users about the effect of dike removal on erosion and increased sedimentation in the river.

"There's a lot of other people potentially impacted by our decision," says Zion resource manager Jeff Bradybaugh. But any water-quality impacts can be mitigated, he says, and in the long run, the proposed project will benefit water users because a free-flowing river will put more water in the floodplain and replenish the water table.