Yakushima has fresh water in abundance. Due to the high mountain rainfall the rivers of Yakushima flow with some of the purest and cleanest water in the world, so there's always a plentiful supply of fresh water to quench your thirst when hiking.
In the warmer months, the river activities you can enjoy on Yakushima include kayaking, swimming, shower climbing and canyoning, as well as just escaping the summer heat, enjoying the refreshingly cool air, having a picnic and enjoying some stunning views.
To help preserve Yakushima's pristine environment please treat the rivers with respect and take care not to pollute them in any way.
Take care when by a river during or after heavy rain or a storm. The rivers can very quickly double in size and become dangerously fast flowing. Snakes are often found near water, but the only one you need to be concerned about on Yakushima is the pit viper (mamushi). Don't be too worried though, because as long as the snake can hear or smell you then it'll quietly disappear unnoticed. Pit vipers are not aggresive and will all they can to disappear without notice.