With biting temperatures no higher than 20 degrees below zero, and winds that would gust to near 80 mph, the four-man climbing team on Mount McKinley had precious little margin for error. And when they tumbled 300-500 feet down "Pig Hill," suffering broken bones and contusions, that margin quickly began to unravel.

A number of national parks are clustered around some of the 10 rivers whose health is being threatened by mining operations, pollution, dredging, proposed dams, and other impacts that American Rivers found them deserving to be listed as America's Most Endangered Rivers.

Yosemite National Park is a landscape of big cliffs, outcrops, and granite domes. But it also is where towering waterfalls, shimmering lakes, and meandering streams mesmerize visitors. In this edition of Yosemite Nature Notes, videographer Steven Bumgardner looks at water in Yosemite.

The year began with a gloomy outlook for travel, with forecasts of $5 per gallon or more for gas all summer. Recent price trends, however, have been downward, with the latest predictions for a national average price of $3.79 per gallon—not much different from 2011. Will the ups and downs of gas prices affect your plans to visit a park this summer?