We happened upon this while up for a wedding in Pierre. It is a free campground! There are RV and tent sites, a dump station, water fill up, bathrooms, and even a park. The tent sites are right on the river and there is even a beach that you can hang out at. You are right on the Lewis and Clark Trail for riding.
The tent sites are pretty secluded for it being a city park. It was a little noisy at night but it was perfect for waking up in the morning and watching the clouds flow over the river with a cup of coffee in hand.