We’re in the process of documenting, in English, around 50 established river and lake touring routes in Hokkaido. We’re not pioneers or first-descenters here. There are a number of guidebooks in Japanese that beautifully document these routes in minute detail. We’re just following their lead.
In the meantime, we can’t recommend the amazing work of HokkaiCamp.com enough. Their information is in Japanese, but they’ve produced great route files nicely annotated with rapids, put in locations etc. Check them out.