Stream (or Gorge) climbing - Sawanobori in Japanese - combines the rivers and the mountains, exercise and getting wet. It involves climbing up rivers, over rocks and through pools and streams, as far as you can go.
Yakushima has many excellent spots for this - the crystal clear water, huge granite rocks, mountain scenery and surrounding forest create the perfect environment.
This activity must be supervised so please take a guide with you. They will also provide you with the right safety equipment, which you must have to safely enjoy this fun activity.
Please take care, the mountains are dangerous. Slopes are steep and paths can be slippery; there are also periodically heavy rainstorms, high winds and typhoons. You may be unlucky enough to experience flash floods, particularly where rivers are narrow. A guide is always recommended, and you should always make sure you have the right equipment.