Kawayu Onsen is located in the town of Teshikaga in Kawakami-gun, Kushiro subprefecture, Hokkaido. The name Kawayu (literally river of hot water) is a loose translation of the Ainu words seseki (hot) and petsu (river), as the river that flows through the center of the spa resort contains hot spring water from Atosanupuri (Mt. Iou).
The spa resort has declared itself "an onsen with 100% non-recycled spring water from source," which is quite rare in Japan, and the famous water is said to provide relief against rheumatism, diabetes and skin disorders.
Kawayu hot spring boasts one of Japan's most highly acidic sources with a pH value of 1.8.
The acidic content of the water washes away the excess oil from the skin, leaving it smooth and healthy.
In addition, the water's abundant sodium content that keeps the body warm after bathing is also a feature that makes this such a unique onsen.