Maybe doesn't have the paddling quality for those on a paddling-only-mission, but is a interesting destination culturally. Rivers are generally short and pool-drop, with a high proportion of dam-release runs. Dave Goadby characterised the rivers as concrete, "even down in the bottom of deep gorges there's pipework and construction stuff".
When to go
How to get there
Fly into Tokyo.
You will need your own vehicle, but the infrastructure is very good and all runs are close to or directly accessible from a road.