Nishi Zawa Kei Koku just happens to be my favorite canyon in all of Japan. Beautiful Caribbean blue water collects in cascading pools all the way down the canyon.
An hour and a half hike from the car park is rewarded in a spectacular 20 meter waterfall. Smooth limestone carved by thousands of years of water flow has rendered perfect slides of all sizes and description.
One of the few canyons in Japan that does not require the use of rope to get down (you can walk, jump, swim or slide down), it was worth braving the September cold for.