We'll let you all in on a secret place we like to go. It's a nice walk down to the Little Sandy River where there used to be a dam that fed the aquafir that filled Rosyln Lake before powering the Bull Run Power Station. Somewhere along the hillside is the exit of the 1/2 mile long tunnel from Marmot Dam on the Big Sandy, but we havn't found it yet. Parts of the bridge are all that remain. It was all removed around 2005, and Rosyln Lake dried up. Lots of childhood memories at that lake.
We like to come down here during the winter to try and stay in some sort of shape. You may encounter BLM, PGE, or Portland Water people during the week, but since nobody knows about this place it's unlikely you will see anyone. It's public land and we've never had anyone tell us to leave. They just smile and wave as they drive back up the hill. It's a 3 mile round trip with about 750 feet of elevation gain (all on the way back). Gps was wonky, but it'll be obvious when you arrive. Cache is in the concrete slabs that were perfect for hiding a cache in.