Kawayu Onsen

The Kumano Kodo pilgrimage trail has been drawing walkers through the Kii Peninsula for over a thousand years, and if you follow the Nakahechi route south from the Hongu Taisha grand shrine, the path eventually brings you down to the banks of the Oto River at Kawayu Onsen. The river itself is the bath here. Not a bath beside the river, not a bathhouse with river views, but the actual river, where hot spring water seeps up directly through the gravel riverbed and mixes with the cold current flowing down from the Yoshino mountains above.

In summer and early autumn you can dig your own pool in the river gravel with a small shovel. The moment you break the surface, warm water percolates in from below at around 73 degrees. You then divert a trickle of the cold river alongside it to bring the temperature down to something comfortable. The hot water carries sodium bicarbonate and sodium chloride pulled from deep volcanic rock, and the mix on your skin is soft and faintly alkaline, the kind of water that leaves you feeling cleaned from inside the pores rather than on top of them. Swimmers who have bathed in rivers all over Japan describe the combination of current and thermal upwelling as something genuinely hard to replicate.

In winter, from December through February, the municipality dams the Oto River with sandbags and creates the Sennin-buro, a communal outdoor pool roughly 40 meters by 15 meters and 60 centimeters deep. The name means the bath of a thousand people. The dam pushes the water level up so the hot spring seeps spread evenly across a huge shallow pool that is technically too large to heat entirely, meaning the temperature varies from patch to patch. You have to explore on your feet to find the warmest pockets. The sky above is wide and dark in the evenings, with no urban light pollution within dozens of kilometers.

Kawayu Onsen village is small, eight or ten ryokan on the road above the riverbank, most of them old and unpretentious. The ryokan baths are good but the free river bathing is what people come for. The Kii Peninsula is not easy to reach from Tokyo or Osaka, but the combination of the ancient pilgrimage roads through cryptomeria forest, the Kumano Taisha shrines, and a river that turns into a hot spring in winter makes the journey feel entirely worth the effort.