The water at Sumatakyo is green — properly green, a colour photographs get accused of inventing — and in November the trees above it turn. The peak runs early to late in the month, wide because the gorge is deep and the upper reaches turn well before the water level does. The valley follows the Sumata, a tributary of the Oi, for about sixteen kilometres.

What people come for sits at the far end of the hot spring village: a suspension footbridge over that green pool. The bridge is the picture, but the walk around it is the better use of the day — the Sumatakyo Promenade Course takes in the bridge and returns in roughly ninety minutes. The hot spring at the end of it is an alkaline simple sulphur water rising from the foot of the Southern Alps.

Getting there is the constraint. The main route runs from Senzu station on the Oigawa Railway, about fifteen kilometres of narrow road along cliffs and through tight curves. Large buses struggle with it. Passing places are limited, so leave time and expect to give way.

Local buses do run from Senzu, not often, and the ride is worth taking for its own sake because the river stays in view from the window. If you drive, go early. The distance, the narrow road, the swaying bridge, the green water — none of it is incidental. The difficulty is part of the place.