Zao Onsen Kawarayu Public Bathhouse

Zao Onsen, steaming across a highland plateau at roughly 880 meters southeast of Yamagata City, is one of Tohoku's oldest hot springs, said to have been opened around 110 AD. Kawarayu Public Bathhouse is one of three communal baths (Kami-yu, Shimo-yu and Kawara-yu) lining Takayu-dori, the retro stone-paved slope running through the heart of the hot spring town. Its name, meaning riverbed bath, comes from the fact that the bathhouse was built directly over a spot where the spring once welled up in the riverbed. The small wooden bathhouse retains the plain, unadorned character of Zao's old healing-bath culture.

The water is an acidic sulfur-bearing aluminum sulfate-chloride spring of the hydrogen sulfide type. According to the analysis certificate posted on site, the source is named Kawarayu Kyodo Yokujo Gensen, with a pH of 1.45 and a source temperature of 48.1 degrees Celsius, making it one of the most strongly acidic springs in Tohoku. What sets it apart is the slatted wooden floor of the tub: the spring wells up directly through the gaps beneath the bather's feet, a rare arrangement known as ashimoto yushutsu. Because the water never touches the air before reaching the bath, it stays clear and pale, and stirring it sends white mineral flakes swirling through the tub. There is no heating, no recirculation and no disinfectant, though a small, constant flow of cold water is fed through a pipe to temper the hot source. The acidity is strong enough to discolor metal, so jewelry should be removed before bathing.

The bathhouse has a single indoor tub each for men and women, with no showers or taps and no shampoo or soap provided. It is unstaffed: bathers drop 300 yen for adults or 100 yen for children into the honesty box at the entrance, and guests staying at Zao Onsen inns can obtain a bathing ticket at their inn's front desk. It is open year-round from 6:00 to 22:00, with an electromagnetic lock closing the door automatically at closing time. There is no dedicated parking lot, so visitors use the public car parks in the hot spring town.