Every so often the lake gets low enough you can see some of what was flooded. Mostly it is tree stumps, but I remember exploring one year when I was in high school, and seeing an old bridge that was collapsed (I think the story is they demolished it prior to completing the dam), and some foundations.

Here's some similar photos (not mine). I think the first one is the bridge I mentioned:

http://www.ghosttownsofwashington.com/kosmos.html