From the wiki: “Kawaji Onsen Hot Spring is a famous onsen [hot spring] district at Nikko near the merging of the Kinugawa and Ojikagawa rivers, with waters said to be especially healing for wounds, injuries and burns. The hot spring was accidentally discovered when the area was flooded in 1723 by Lake Ikari-ko, after a long period of rain, when a hot spring was discovered gushing up from the riverbed after the floodwaters subsided.
“Nestled in this tranquil mountain environment, the riverside open-air baths allow visitors to enjoy an idyllic rustic onsen while also being able to admire the beauty of mountain stream scenery. The Kinugawa river itself is somewhat unusual in Japan. The name is made up of three characters: ‘demon’, ‘rage’ and ‘river’, and there are various tales regarding the origin of the name including a story that the river was dubbed ‘the river of the raging demon’ for when its usually gently flowing waters temporarily swell and become rough, they rage violently like a demon in a fury.”