A night exposure of Old Faithful Geyser, the first geyser in the park to be named, erupting in Yellowstone National Park, Wyoming, June 23, 2011. Currently the geyser erupts approximately every 90 minutes. REUTERS/Jim Urquhart (UNITED STATES)

A herd of bison swim across the Yellowstone River in Yellowstone National Park, Wyoming, June 21, 2011. On average over 3,000 bison live in the park.REUTERS/Jim Urquhart (UNITED STATES)

A grizzly bear and her cub in the Hayden Valley in Yellowstone National Park, Wyoming, June 24, 2011. Grizzly bears are most often found in the open spaces of the park. REUTERS/Jim Urquhart (UNITED STATES)

Grizzly bear cubs play on snow in the Hayden Valley in Yellowstone National Park, Wyoming, June 24, 2011. Grizzly bears are most often found in the open spaces of the park. REUTERS/Jim Urquhart (UNITED STATES)

A wolf walks with a set of elk legs in the Hayden Valley after killing an elk in Yellowstone National Park, Wyoming, June 20, 2011. Elk are the most common food of the wolves in the park. REUTERS/Jim Urquhart (UNITED STATES)

A herd of bison graze in Lamar Valley in Yellowstone National Park, Wyoming, June 20, 2011. On average over 3,000 bison live in the park. REUTERS/Jim Urquhart (UNITED STATES)

A rainbow of light can be seen under the right conditions at the base of the 93 meter tall Yellowstone River Lower Falls in Yellowstone National Park, Wyoming, June 21, 2011. REUTERS/Jim Urquhart (UNITED STATES)

A black bear roams near the Lamar Valley in Yellowstone National Park, Wyoming, June 24, 2011. Black bear are commonly sighted near the tree line. REUTERS/Jim Urquhart (UNITED STATES)

The view of the Absaroka Mountain Range of the north eastern stretch of park at sunrise in Yellowstone National Park, Wyoming, June 22, 2011. REUTERS/Jim Urquhart (UNITED STATES)

A coyote hunts in a the Lamar Valley in Yellowstone National Park, Wyoming, June 20, 2011. Coyotes have slender faces, pointy ears, are much smaller than wolves but are often mis-identified as wolves by tourist in the park. REUTERS/Jim Urquhart (UNITED STATES)

A coyote hunts in a the Lamar Valley in Yellowstone National Park, Wyoming, June 20, 2011. Coyotes have slender faces, pointy ears, are much smaller than wolves but are often mis-identified as wolves by tourist in the park. REUTERS/Jim Urquhart (UNITED STATES)

The Grand Prismatic Spring, the largest in the United States and third largest in the world, and it's colored bacteria and microbial mats in Yellowstone National Park, Wyoming, June 22, 2011. REUTERS/Jim Urquhart (UNITED STATES)

The Grand Prismatic Spring, the largest in the United States and third largest in the world, and it's colored bacteria and microbial mats in Yellowstone National Park, Wyoming, June 22, 2011. REUTERS/Jim Urquhart (UNITED STATES)

A cinnamon black bear, a subspecies of the American black bear, runs between cars near Madsion in Yellowstone National Park, Wyoming, June 19, 2011. REUTERS/Jim Urquhart (UNITED STATES)

Artist Jinsheng Song paints the Lower Yellowstone Falls in Yellowstone National Park, Wyoming, June 22, 2011. REUTERS/Jim Urquhart (UNITED STATES)

The Shell Spring erupts in the Biscuit Basin in Yellowstone National Park, Wyoming, June 23, 2011. REUTERS/Jim Urquhart (UNITED STATES)

The sunrise is reflected in the Yellowstone River in Yellowstone National Park, Wyoming, June 24, 2011. REUTERS/Jim Urquhart (UNITED STATES)

A bison emerges after swimming across the Yellowstone River in Yellowstone National Park, Wyoming, June 21, 2011. On average over 3,000 bison live in the park. REUTERS/Jim Urquhart (UNITED STATES)

A black bear swims in Rainy Lake near Tower Fall in Yellowstone National Park, Wyoming, June 20, 2011. Black bear are commonly sighted near the tree line. REUTERS/Jim Urquhart (UNITED STATES)

A black bear rubs itself against a tree near Rainy Lake near Tower Fall in Yellowstone National Park, Wyoming, June 24, 2011. Black bear are commonly sighted near the tree line. REUTERS/Jim Urquhart (UNITED STATES)

Boulders deposited by a glacial icecap dot the Lamar Valley in Yellowstone National Park, Wyoming, June 23, 2011. REUTERS/Jim Urquhart (UNITED STATES)

A wolf rolls in the snow in the Hayden Valley after killing and eating an elk in Yellowstone National Park, Wyoming, June 20, 2011. Elk are the most common food of the wolves in the park. REUTERS/Jim Urquhart (UNITED STATES)