Seeley Lake's dam was washed out during the massive rainstorms in August 2011. The dam was privately owned and had county road 617 running on it. For whatever reason the dam was never rebuilt and is now just a bridge.
This is an easy enough guard rail grab, plenty of safe parking on the corner of Columbia Hiway/617 and Holding Rd. No need to stand on the road!
Be safe and have fun!
Caution: hint gives it away!