Pecatonica River Forest Preserve is located near Rockford, IL. The trail is primarily used for birding and is accessible from April until September.
Pecatonica River Forrest Preserve is a county park with a fair number of campsites, plus group camping along the river. The river meanders quite a bit, during the spring it is prone to flooding the field where the group camping is.
I camped here probably over a dozen times as a youth in the BSA as well as with family. Besides the campground there are a few nice trails to take. One runs along an oxbow swamp, it has a nice observation deck to watch the wildlife from over the oxbow swamp / pond.
In the other direction of the oxbow swamp there is a trail which runs through a hilly sandprarie. This sandprarie is a fairly unique ecosystem which is home to a number of endangered species.
The trail map can be found here: