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The Firehole River flows through a canyon in Yellowstone National Park in Wyoming, USA The Firehole River flows north 21 miles from its source in Madison Lake on the Continental Divide to join the Gibbon River to form the Madison River. The Firehole flows through several significant geyser basins in the park. The river was named by early trappers for the steam that makes it appear to be smoking as if on fire...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...Firehole River has been a fishing mecca since the late 19th century and is known today for its excellent fly fishing.