That's just a big lake.
I look up at the bench that marked the ancient shoreline and feel awe to realize how much water was here once upon a time.
It was a BIG lake.
And even after the water broke out and ran down the snake river channel, it didn't drain all of the lake...the change in climate did that.
Things to meditate on, when you realize how things do really change.
Here's a geological description of the event. Whoa!
http://imnh.isu.edu/digitalatlas/hydr/lkbflood/lbf.htm
One of the high points of my childhood was finding fossils in the foothills. Most of them were just lying on the ground, or could be had with very little digging.