This is located in the hills behind Kitakyushu in the same range that Mt Sarakura is located. This picture is actually taken from the top of Mt Sarakura.
It is possible to walk or cycle around the reservoir and there is an interesting folk village store you can look at. Nearby a new spa was recently built.
To get to Kawachi you can take a bus from Kokura Station (number 1 bus is best) to Okura. From there you must change to another bus. There are direct buses but I forget the number!