The 535-sq-km Shannon National Park is on the South Western Hwy, 53km south of Manjimup. Until 1968 Shannon was the site of WA's biggest timber mill. Areas of this park were badly damaged by forest fires in 2015, and at the time of writing access to the park was not possible. Check with the Department of Parks & Wildlife online or in Pemberton on the current situation.
The 48km Great Forest Trees Drive is a one-way loop, split by the highway. Start at the park day-use area on the north of the highway. From here there's an easy 3.5km walk to the Shannon Dam and a steeper 5.5km loop to Mokare's Rock, with a boardwalk and great views.
Further along, the 8km (return) Great Forest Trees Walk crosses the Shannon River. Off the southern part of the drive, boardwalks look over stands of giant karri at Snake Gully and Big Tree Grove.
In the park's southwest, a 6km (return) walking track links Boorara Tree with a lookout point over Lane Poole Falls.
There is a sizeable campground with showers in the spot where the original timber-milling town used to be. Following the fires in 2015 it was closed for redevelopment at the time of writing. Check with Department of Parks & Wildlife in Pemberton on the current status.