Male elk bugling during the fall rut. Large male elk are known as bulls. Male elk have large antlers which are shed each year. Male elk engage in competitive mating behaviors during the rut, including posturing, antler wrestling and bugling, a loud series of screams which is intended to establish dominance over other males and attract females, Cervus canadensis, Mammoth Hot Springs, Yellowstone National Park, Wyoming

Male elk bugling during the fall rut, Yellowstone National Park, Wyoming. Cervus canadensis.

Bull elk spar to establish harems of females, Gibbon Meadow, Cervus canadensis, Gibbon Meadows, Yellowstone National Park, Wyoming

Elk, Gibbon Meadows

A rainbow appears in the mist of the Lower Falls of the Yellowstone River.  At 308 feet, the Lower Falls of the Yellowstone River is the tallest fall in the park.  This view is from Lookout Point on the North side of the Grand Canyon of the Yellowstone.  When conditions are perfect in midsummer, a midmorning rainbow briefly appears in the falls, Yellowstone National Park, Wyoming

Rainbow, Lower Yellowstone Falls, Grand Canyon Of The Yellowstone

Grand Prismatic Spring (left) and Excelsior Geyser (right).  Grand Prismatic Spring displays a stunning rainbow of colors created by species of thermophilac (heat-loving) bacteria that thrive in narrow temperature ranges.  The blue water in the center is too hot to support any bacterial life, while the outer orange rings are the coolest water.  Grand Prismatic Spring is the largest spring in the United States and the third-largest in the world.  Midway Geyser Basin, Yellowstone National Park, Wyoming

Grand Prismatic Spring, Midway Geyser Basin

Old Faithful geyser, sunrise.  Reaching up to 185' in height and lasting up to 5 minutes, Old Faithful geyser is the most famous geyser in the world and the first geyser in Yellowstone to be named, Upper Geyser Basin, Yellowstone National Park, Wyoming

Old Faithful, Yellowstone National Park, Wyoming.

Mule deer in tall grass, fall, autumn, Odocoileus hemionus, Yellowstone National Park, Wyoming

Mule Deer

The bisons massive head is its most characteristic feature. Its forehead bulges because of its convex-shaped frontal bone. Its shoulder hump, dwindling bowlike to the haunches, is supported by unusually long spinal vertebrae. Over powerful neck and shoulder muscles grows a great shaggy coat of curly brown fur, and over the head, like an immense hood, grows a shock of black hair. Its forequarters are higher and much heavier than its haunches. A mature bull stands about 6 1/2 feet (2 meters) at the shoulder and weighs more than 2,000 pounds (900 kilograms). The bisons horns are short and black. In the male they are thick at the base and taper abruptly to sharp points as they curve outward and upward; the females horns are more slender, Bison bison, Yellowstone National Park, Wyoming

American Bison

Fly fishing below Gibbon Falls. This flyfisherman hiked up the Gibbon River to reach the foot of Gibbon Falls, Yellowstone National Park, Wyoming

Flyfishing At Gibbon Falls, Gibbon River

A rainbow appears in the spray of Riverside Geyser as it erupts over the Firehole River.  Riverside is a very predictable geyser.  Its eruptions last 30 minutes, reach heights of 75 feet and are usually spaced about 6 hours apart.  Upper Geyser Basin, Yellowstone National Park, Wyoming

Riverside Geyser And Rainbow, Upper Geyser Basin

Male elk bugling during the fall rut. Large male elk are known as bulls. Male elk have large antlers which are shed each year. Male elk engage in competitive mating behaviors during the rut, including posturing, antler wrestling and bugling, a loud series of screams which is intended to establish dominance over other males and attract females, Cervus canadensis, Madison River, Yellowstone National Park, Wyoming

Elk, Madison River

Grizzly bear, autumn, fall, brown grasses, Ursus arctos horribilis, Lamar Valley, Yellowstone National Park, Wyoming

Grizzly Bear, Lamar Valley

Male elk bugling during the fall rut. Large male elk are known as bulls. Male elk have large antlers which are shed each year. Male elk engage in competitive mating behaviors during the rut, including posturing, antler wrestling and bugling, a loud series of screams which is intended to establish dominance over other males and attract females, Cervus canadensis, Madison River, Yellowstone National Park, Wyoming

Elk Bugling, Madison River

Old Faithful geyser.  Reaching up to 185' in height and lasting up to 5 minutes, Old Faithful geyser is the most famous geyser in the world and the first geyser in Yellowstone to be named, Upper Geyser Basin, Yellowstone National Park, Wyoming

Old Faithful geyser, Yellowstone National Park

Coyote in snow covered field along the Madison River, Canis latrans, Yellowstone National Park, Wyoming

Coyote

Yellow-bellied marmots can often be found on rocky slopes, perched atop boulders, Marmota flaviventris, Yellowstone National Park, Wyoming

Yellow-bellied Marmot

Morning Glory Pool has long been considered a must-see site in Yellowstone.  At one time a road brought visitors to its brink.  Over the years they threw coins, bottles and trash in the pool, reducing its flow and causing the red and orange bacteria to creep in from its edge, replacing the blue bacteria that thrive in the hotter water at the center of the pool.  The pool is now accessed only by a foot path.  Upper Geyser Basin, Yellowstone National Park, Wyoming

Morning Glory Pool, Upper Geyser Basin

A crowd enjoys watching Old Faithful geyser at peak eruption, Upper Geyser Basin, Yellowstone National Park, Wyoming

Old Faithful Geyser, Yellowstone, Upper Geyser Basin

A visitor videotapes the eruption of Lion Geyser, with Old Faithful Inn visible in the distance.  Lion Geyser, whose eruption is preceded by a release of steam that sounds like a lion roaring, erupts just once or a few times each day, reaching heights of up to 90 feet.  Upper Geyser Basin, Yellowstone National Park, Wyoming

Lion Geyser, Upper Geyser Basin

Castle Geyser erupts, reaching 60 to 90 feet in height and lasting 20 minutes.  While Castle Geyser has a 12 foot sinter cone that took 5,000 to 15,000 years to form, it is in fact situated atop geyserite terraces that themselves may have taken 200,000 years to form, making it likely the oldest active geyser in the park. Upper Geyser Basin, Yellowstone National Park, Wyoming

Castle Geyser, Upper Geyser Basin

Grand Geyser erupts (right) with a simultaneous eruption from Vent Geyser (left).  Grand Geyser is a fountain-type geyser reaching 200 feet in height and lasting up to 12 minutes.  Grand Geyser is considered the tallest predictable geyser in the world, erupting about every 12 hours.  It is often accompanied by burst or eruptions from Vent Geyser and Turban Geyser just to its left.  Upper Geyser Basin, Yellowstone National Park, Wyoming

Grand Geyser, Upper Geyser Basin

Grand Prismatic Spring displays a stunning rainbow of colors created by species of thermophilac (heat-loving) bacteria that thrive in narrow temperature ranges.  The blue water in the center is too hot to support any bacterial life, while the outer orange rings are the coolest water.  Grand Prismatic Spring is the largest spring in the United States and the third-largest in the world.  Midway Geyser Basin, Yellowstone National Park, Wyoming

Grand Prismatic Spring, Midway Geyser Basin

Grand Prismatic Spring displays brilliant colors along its edges, created by species of thermophilac (heat-loving) bacteria that thrive in narrow temperature ranges.  The outer orange and red regions are the coolest water in the spring, where the overflow runs off.  Midway Geyser Basin, Yellowstone National Park, Wyoming

Grand Prismatic Spring, Midway Geyser Basin

Steam rises above the Midway Geyser Basin, largely from Grand Prismatic Spring and Excelsior Geyser. The Firehole River flows by, Yellowstone National Park, Wyoming

Grand Prismatic Spring And Excelsior Geyser, Midway Geyser Basin

New Blue Spring and its travertine terraces, part of the Mammoth Hot Springs complex, Yellowstone National Park, Wyoming

Travertine Terraces, New Blue Spring, Mammoth Hot Springs

Bison, Bison bison, Yellowstone National Park, Wyoming

American Bison

Yellowstone Falls from Uncle Tom's Trail.  Lower Yellowstone Falls shows a beautiful rainbow as it cascades 308' in a thundering plunge into the Grand Canyon of the Yellowstone River, Yellowstone National Park, Wyoming

Yellowstone Falls From Uncle Tom's Trail, Grand Canyon Of The Yellowstone

Excelsior Geyser, now dormant, was formerly the worlds largest geyser. It still produces immense runoff into the Firehole River: 4,500 gallons per minute, or 6 million gallons per day. It is located in Midway Geyser Basin, Yellowstone National Park, Wyoming

Excelsior Geyser, Midway Geyser Basin

Madison River steaming in the cold air, sunrise, autumn, tall grasses and golden light, Yellowstone National Park, Wyoming

Madison River steaming in the cold air, Yellowstone National Park.

Bacteria mats and Grand Prismatic Spring.  The orange color is due to bacteria which thrive only on the cooler fringes of the hot spring, while the hotter center of the spring hosts blue-colored bacteria, Midway Geyser Basin, Yellowstone National Park, Wyoming

Bacteria Mats And Grand Prismatic Spring, Midway Geyser Basin

Wildflowers on Mount Washburn, on the north side of Dunraven Pass near Tower Junction, Helianthella uniflora, Yellowstone National Park, Wyoming

Wildflowers On Mount Washburn

Steam rises at sunrise in Norris Geyser Basin.  Located at the intersection of three tectonic faults, Norris Geyser Basin is the hottest and most active geothermal area in Yellowstone National Park

Steam Rises At Sunrise In Norris Geyser Basin

Steam rises at sunrise in Norris Geyser Basin.  Located at the intersection of three tectonic faults, Norris Geyser Basin is the hottest and most active geothermal area in Yellowstone National Park

Steam rises at sunrise in Norris Geyser Basin. Yellowstone National Park.

Gibbon River meanders through Gibbon Meadows, sunrise and clouds reflected in the calm waters, Yellowstone National Park, Wyoming

Gibbon River meanders through Gibbon Meadows, sunrise and clouds reflected in the calm waters, Wyoming.

Fly fisherman wading in the Madison River, fall, autumn, Yellowstone National Park, Wyoming

Fly Fisherman Wading In The Madison River

Elk rest in tall grass during the midday heat, Gibbon Meadow, Cervus canadensis, Gibbon Meadows, Yellowstone National Park, Wyoming

Elk, Gibbon Meadows

The Yellowstone River flows through the Hayden Valley, Yellowstone National Park, Wyoming

Yellowstone River, Hayden Valley

Castle Geyser erupts with the colorful bacteria mats of Tortoise Shell Spring in the foreground.  Castle Geyser reaches 60 to 90 feet in height and lasts 20 minutes.  While Castle Geyser has a 12 foot sinter cone that took 5,000 to 15,000 years to form, it is in fact situated atop geyserite terraces that themselves may have taken 200,000 years to form, making it likely the oldest active geyser in the park. Upper Geyser Basin, Yellowstone National Park, Wyoming

Tortoise Shell Spring And Castle Geyser, Upper Geyser Basin

Grand Geyser (right), Turban Geyser (center) and Vent Geyser (left) erupt in concert.  An apron of bacteria covered sinter occupies the foreground when water from the eruptions flows away.  Grand Geyser is a fountain-type geyser reaching 200 feet in height and lasting up to 12 minutes.  Grand Geyser is considered the tallest predictable geyser in the world, erupting about every 12 hours.  It is often accompanied by burst or eruptions from Vent Geyser and Turban Geyser just to its left.  Upper Geyser Basin, Yellowstone National Park, Wyoming

Grand Geyser, Upper Geyser Basin

Yellowstone National Park, entrance sign at southern entrance, Snake River is visible in the background

Steam rises in the Porcelain Basin, Norris Geyser Basin, Yellowstone National Park, Wyoming

Norris Geyser Basin

A rainbow appears in the spray of Pink Cone Geyser.  Pink Cone Geyser reaches 30 feet in height, and has highly variable interval and duration.  It is a cone-type geyser and its cone has a pinkish tint due to manganese oxide in it.  Firehole Lake Drive, Lower Geyser Basin, Yellowstone Park, Yellowstone National Park, Wyoming

Pink Cone Geyser, Lower Geyser Basin

Firehole Lake creates a wall of steam in the early morning, Lower Geyser Basin, Yellowstone National Park, Wyoming

Firehole Lake creates a wall of steam in the early morning.

New Blue Spring steams in the cold morning air with Mammoth Hot Springs Inn in the distance, Yellowstone National Park, Wyoming

New Blue Spring Steaming, Mammoth Hot Springs

Burned trees in grass meadow in Lower Geyser Basin.  Grass on the left has hot runoff from nearby thermal springs, keeping it free of snow, Yellowstone National Park, Wyoming

Sheepeater Cliffs, an example of columnar jointing in basalt due to shrinkage during cooling, Yellowstone National Park, Wyoming

Sheepeater Cliffs

Dead trees embedded in calcium carbonate deposits in the travertine terraces of Mammoth Hot Springs, near Minerva terrace .  Over two tons of calcium carbonate (in solution) is deposited each day on the terraces, gradually killing any vegetation that had managed to be growing, Yellowstone National Park, Wyoming

Dead Trees, Minerva Terrace, Mammoth Hot Springs