How did it form??
It began as a River of Fire when basaltic lava flowed from fissures in the earth's surface and traveled long distances through lava tubes. Lava tubes develop when the upper surface of a basalt flow rapidly cools, causing the outer edges to solidify while the hot inner lava continues to flow underneath. As the molten lava drains out it leaves behind a long hollow tube. Eventually the River of Fire is replaced by a River of Water.
The Rogue River entered a section of a lava tube, becoming a "hidden river" which flows below ground for 200 feet.
In doing so, it created a "natural bridge" that connects the banks of the river. Indians and early settlers used this natural bridge to cross the Rogue River.
The Rogue River emerges from the lava tube outlet after a short underground trip and continues its journey to the Pacific Ocean 184 miles downstream. At peak flow approximately 335,000 gallons of water rush from this outlet at the speed of 6 feet per second. Fed by the melting snow on the slopes of Mt. Mazama (Crater Lake) in the High Cascades, the river's temperature at the Natural Bridge varies seasonally from 32 to 69 degrees F.