In the fall of 2007 we happened onto Steamboat Rock while on a tour to see the Grand Coulee Dam. Occupying about two square miles, the Rock sits in what is now Banks Lake. See: http://eruttledge.zenfolio.com/p492004455/h13ea1d4#h13ea1d4
for the photo I posted from that visit.
This year I returned to hike to the top of this amazing natural structure. The trail is short, but includes a steep scree-riddled ascent. Nevertheless, the view from the top was spectacular.