It’s Riffe* Lake. And it’s because there is the ruins of a town called Riffe* at the bottom, including a cemetery.
EDIT: spelling.

It's Riffe Lake. And there are at least 3 towns down there (Riffe, Nesika, and Kosmos).

Thank you for that correction. I’m not originally from Washington, and that’s an amazing fact that I didn’t know about this lake!!!!

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

Yeah I’ve seen it, I live not only 5 min away, and I have to say it’s some of the best views I’ve ever seen in my life!!!!!