With a surface area of 136 square miles, Yellowstone Lake is the largest lake at high elevation (i.e., more than 7,000 ft.) in North America. It is a natural lake, situated at 7,733 ft. above sea level. It is roughly 20 miles long and 14 miles wide with 110 miles of shoreline. It is frozen nearly half the year. It freezes in late December or early January and thaws in late May or early June.
The Yellowstone River is the last major undammed river in the lower 48 states, flowing 671 miles from its source southeast of Yellowstone into the Missouri River and then, eventually, into the Atlantic Ocean. It begins in the Absaroka Mountain Range on Yount Peak. The river enters the park and meanders through the Thorofare region into Yellowstone Lake. It leaves the lake at Fishing Bridge and flows north over LeHardy Rapids and through Hayden Valley. After this peaceful stretch, it crashes over the Upper and Lower falls of the Grand Canyon. It then flows generally northwest, meeting its largest tributary, the Lamar River, at Tower Junction. It continues through the Black Canyon and leaves the park near Gardiner, Montana. The Yellowstone River continues north and east through the state of Montana and joins the Missouri River near the eastern boundary line of the state. The Missouri River eventually joins the Mississippi River, which flows into the Atlantic Ocean at the Gulf of Mexico.
In addition to the Yellowstone River, many of the spawning streams in the Lake/Fishing Bridge/Bridge Bay area provide critical food sources for grizzly bears in the spring time. Therefore, ecologically speaking, these river and streams are a primary resource in the district. The LeHardy Rapids are a cascade on the Yellowstone River, three miles north of Fishing Bridge. Geomorphologically, it is thought that this is the actual spot where the lake ends and the river continues its northward flow. In the spring, many cutthroat trout may be seen here, resting in the shallow pools before expending bursts of energy to leap up the rapids on the their way to spawn under Fishing Bridge.
The rapids were named for Paul LeHardy, a civilian topographer with the Jones Expedition in 1873. Jones and a partner started off on a raft with the intent of surveying the river, planning to meet the rest of their party at the Lower Falls. Upon hitting the rapids, the raft capsized, and many of the supplies were lost, including guns, bedding, and food. LeHardy and his partner saved what they could and continued their journey to the falls on foot.
The rapids became a popular visitor attraction when a boardwalk was built in 1984 providing access to the area. Due to increased visitation, a group of harlequin ducks, which once frequented this area in spring, have not been seen for several years. The boardwalk has consequently been closed in early spring to protect this sensitive habitat, but the harlequins have not returned.
When the Washburn Expedition explored the area in 1870, Nathaniel Langford described Mud Volcano as greatest marvel we have yet met with. Although the Mud Volcano can no longer be heard from a mile away nor does it throw mud from its massive crater, the area is still eerily intriguing.
The short loop from the parking lot past the Dragon's Mouth and the Mud Volcano is handicapped accessible. The half-mile upper loop trail via Sour Lake and the Black Dragon's Caldron is relatively steep. Two of the most popular features in the Mud Volcano front country are the Dragon's Mouth and the Black Dragon's Caldron. The rhythmic belching of steam and the flashing tongue of water give the Dragon's Mouth Spring its name, though its activity has decreased notably since December 1994. The Black Dragon's Caldron exploded onto the landscape in 1948, blowing trees out by their roots and covering the surrounding forest with mud. The large roil in one end of the Caldron gives one the sense that the Black Dragon itself might rear its head at any time.
In January 1995, a new feature on the south bank of Mud Geyser became extremely active. It covers an area of 20 by 8 feet and is comprised of fumaroles, small pools, and frying-pan type features. Much of the hillside to the south and southwest of Mud Geyser is steaming and hissing with a few mudpots intermixed. This increase in activity precipitated a great deal of visitor interest and subsequent illegal entry into the area.
The most dramatic features of the Mud Volcano area however, are not open to the public. The huge seething mud pot known as the Gumper is located off-boardwalk behind Sour Lake. The more recent features just south of the Gumper are some of the hottest and most active in the area. Ranger-guided walks are offered to provide visitors an opportunity to view this interesting place.
Farther in the backcountry behind Mud Volcano, several features are being tested for the existence of thermophilic microbes, which may offer insights into origin of life theories as well as having medical/environmental applications.
The Sulphur Caldron area can be viewed from a staging area just north of Mud Volcano. The Sulphur Caldron is among the most acidic springs in the park with a pH of 1.3. Its yellow, turbulent splashing waters bring to mind images of Shakespeare's soothsayers. Other features which can be viewed from this overlook are Turbulent Pool (which is no longer turbulent) and the crater of a large, active mudpot.
For more specific information on the features of the Mud Volcano/Sulphur Caldron area, consult the annual reports that are available in the Ranger Naturalist Office adjacent to the Fishing Bridge Visitor Center.
The Hayden Valley is located six miles north of Fishing Bridge Junction. The Pelican Valley is situated three miles east of Fishing Bridge. These two vast valleys comprise some of the best habitat in the lower 48 states for grizzly bears, bison, elk, and other wildlife species.
Located just south of Bridge Bay Campground, it is an easy one-mile walk to the Natural Bridge. There is also a bicycle trail leading to the bridge. The Natural Bridge was formed by erosion of this rhyolite outcrop by Bridge Creek. The top of the bridge is approximately 51 ft. above the creek. A short switchback trail leads to the top, though travel across the bridge is now prohibited to protect this feature.