The Firehole River is one of two major tributaries of the Madison River. It flows north approximately 21 miles (34 km) from its source in Madison Lake on the Continental Divide to join the Gibbon River at Madison Junction in Yellowstone National Park. The Firehole flows through several significant geyser basins in the park to include the Upper Geyser Basin, which contains the world-famous geyser Old Faithful. The river was named by early trappers for the steam that makes it appear to be smoking as if on fire.
The Firehole contains three of Yellowstone's major waterfalls: Kepler Cascades south of Old Faithful, Firehole Falls and the Cascades of the Firehole in Firehole Canyon.
The river is surrounded by geothermal features which empty water into it. One effect of the input of this water is to increase the temperature of the water. Temperatures in the river have been measured as high as 30 °C (86 °F) and average 5 to 10 °C (9 to 18 °F) higher than areas upstream of geothermal influence.
Water entering the river from geothermal features contains dissolved chemicals and minerals. Levels of boron and arsenic have been found to above the standard limits for protection of aquatic organisms. Despite these levels, rainbow trout live and spawn in these waters.
Firehole River has been a fishing mecca since the late 19th century and is known today for its excellent fly fishing.
The River is also home to an invasive species, the New Zealand mud snail (Potamopyrgus antipodarum), that is being heavily studied to determine its impact on the river's ecosystem.