During Edo period (early 17th to mid-19th centuries) local daimyos encouraged swimming as part of samurai children training. 'Shinderyu' 神伝流 is one of the dozen or so native swimming styles developed prior to the adoption of western methods.
Defensive techniques are performed wearing full military gear, while taking advantage of the river stream. Fundamentally, the purpose of this method was to facilitate the crossing of deep river currents during the Warring States period. The Hiji River in Ozu is one of the few watercourses in Japan where this suijutsu style has been preserved up to this day.
Early January, on Seijin no Hi -Coming of Age Day-, local kids and adults gather on the banks of the Hiji River and dip in freezing waters to perform their waza routines.