William Shirakawa is quite diverse despite its short length, from its silent birth in the springs of Aso to its broad flow into Ariake Sea. A delightful area many overlook is its passage through Nishihara Mura, where a beautiful waterfall, Shiroito (“White Thread”), cascades 20 meters down volcanic rock hundreds of thousands of years old. Volcanic columns formed when lava cooled and solidified are apparent. The waterfall has been selected as one of the 100 best waters in Kumamoto Prefecture. Heavily forested, the spot is an oasis from summer heat, is not too far from the city, and embodies the three symbols of Kumamoto: red, from lava; green, from forest; and blue, from water. A pool at the fall’s foot allows for a dip, though be considerate of photographers. NOTE: Douglas Bruce comments below on the course of the river following the falls: it is the “Taki river which flows into the Kiyama river. The Kiyama river becomes the Futa river for a short stretch and then retakes the name of Kiyama river. The Kiyama river flows on into the Kaze river, which passes the southern end of Ezu lake. Finally the Kaze river joins the Midorikawa and then flows into the Ariake sea.” Thank you, Douglas.
This useful site contains many photos and videos of the area:
https://nishiharakanko.com/shiraitonotaki/