We drove through here ages back while exploring. Onsen areas often look a bit run down, but this place really has a gone bust look to it. There are a few broken down onsens there slowly going to seed. We never thought to go there.
Anyway, by chance we ended up spending a midweek night at Myougaya Honkan (the remaining ryokan). Wow! What a great place. I don't think we really got the photos to do it justice.
There are two distinct parts to Myougaya Honkan. The main building is a modern building and unremarkable. Our rooms were here and they were also pretty standard ryokan/hotel rooms. Normal tatami rooms with futon beds. The view from the rooms down onto the river was very good though.
The rest of the complex is old wooden buildings. The dining area is here. Also there is a covered stairway down to the onsen area.
The food was first rate. Fairly simple kaiseki. The best were the beef cooked on a hotplate and some sashimi (aiyu I think).
The highlight was the riverside onsen. There are men's and women's indoor hot tubs. When we were there the outdoor onsen was mixed in the evening and women only in the morning (6AM - 8AM). There are different pools of different temperature. You are right by the river and can dip your feet in it if you like.
Midweek, peak season (they told us its more expensive when we went in November) it cost 9000 yen each for 4 to a room (I think 2 to a room is 11k each).