A rainbow appears at the base of the Yellowstone River Lower Falls in Yellowstone National Park, Wyoming, on June 21, 2011. #

A coyote hunts in a the Lamar Valley in Yellowstone National Park, Wyoming, on June 20, 2011. #

A tree sits on a submerged island in Yellowstone Lake in Yellowstone National Park, on June 21, 2011. #

Tourists watch the "Old Faithful" geyser, which erupts on average every 90 minutes, in Yellowstone National Park, on June 1, 2011. #

A herd of elk travels through the Hayden Valley in Yellowstone National Park, Wyoming, on June 21, 2011. #

A blue-billed ruddy duck lands in Floating Island Lake in Yellowstone National Park, on June 24, 2011. #

Grizzly bear cubs play on the snow in the Hayden Valley in Yellowstone National Park, on June 24, 2011. Picture taken June 24, 2011. #

The Grand Prismatic Spring, the largest in the United States and third largest in the world, in Yellowstone National Park, photographed on June 22, 2011. #

The view of the Absaroka Mountain Range of the north eastern stretch of park at sunrise in Yellowstone National Park, on June 22, 2011. #

Bison swim across the Yellowstone River in Yellowstone National Park, on June 21, 2011. #

A bison emerges after swimming across the Yellowstone River, on June 21, 2011. #

Visitors watch for wolves in the Hayden Valley in Yellowstone National Park, on June 20, 2011. #

A Chipmunk stretches while begging for food in Yellowstone National Park, on June 2, 2011. #

Trees line the bank of the Yellowstone River in Yellowstone National Park, on June 21, 2011. #

A fly fisherman on the Firehole River in Yellowstone National Park, Wyoming, on June 21, 2011. #

A wolf walks with a pair of elk legs in the Hayden Valley, after killing and eating the elk, in Yellowstone National Park, on June 20, 2011. #

The Grand Prismatic Spring, in Yellowstone National Park, photographed on June 22, 2011. #

A closer view of Yellowstone's Grand Prismatic Spring. Picture taken June 22, 2011. #

A stream meanders through the Hayden Valley in Yellowstone National Park, on June 21, 2011. #

A black bear swims in Rainy Lake, near Tower Falls, in Yellowstone National Park, on June 20, 2011. #

Steam rises from geysers and thermal features in the Biscuit Basin in Yellowstone National Park, on June 23, 2011. #

Old Faithful Geyser erupts in Yellowstone National Park, Wyoming, on June 22, 2011. #

A herd of bison grazes in the Lamar Valley in Yellowstone National Park, on June 20, 2011. #

The shadow of a pine tree on a eroding cliff face composed of columnar basalt, near Tower Falls, in Yellowstone National Park, on June 22, 2011. #

A bull elk with velvet still on its antlers grazes near Madison in Yellowstone National Park, on June 19, 2011. #

View of the Lower Falls at the Yellowstone Grand Canyon in the Yellowstone National Park, on June 2, 2011. #

People stand at the brink of the 93-meter tall Yellowstone River Lower Falls in Yellowstone National Park, on June 24, 2011. #

Water flows on the ever changing travertine-depositing Mammoth Hot Springs in Yellowstone National Park, on June 22, 2011. #

A calf in a herd of bison in the Lamar Valley in Yellowstone National Park, on June 22, 2011. On average over 3,000 bison live in the park. #

Silica from geysers (front) and the partially frozen Yellowstone Lake (back) at the West Thumb Geyser Basin in the Yellowstone National Park, on June 2, 2011. #

A black bear walks across a meadow near Rainy Lake in Yellowstone National Park, on June 20, 2011. #

Boiling mud and water in Yellowstone National Park, on June 2, 2011. #

A night exposure of Old Faithful Geyser erupting in Yellowstone National Park, on June 23, 2011. #

A fox hunts near Tower Falls in Yellowstone National Park, on June 24, 2011. #

Boulders deposited long ago by glaciers dot the Lamar Valley in Yellowstone National Park, photographed on June 23, 2011. #

A black bear runs across a road bridge spanning the Yellowstone River near Lamar Valley in Yellowstone National Park, on June 24, 2011. #