Miho Dam is near Minamiashigarashi and you can reach it by car. It is a very nice drive. You will drive through the countryside of Ashigara Pass, through Jizodo and Yamakita (you may want to stop in Jizodo) to see Yuhi Falls (Yuhinotaki).
Yuhinotaki from Wikipedia
From Yamakita, Miho Dam is about 20 minutes by car. It’s beautiful! See the photos below: