As I type this, we're camped at Henry's Lake State Park in northeastern Idaho - about 15 miles west of Yellowstone. A couple of days ago we took the Mesa Falls Scenic Byway, and came to Upper Mesa Falls. Henry's Lake and the Island Park area are in a giant caldera, similar to the one at Yellowstone. The Henry's Fork (of the Snake) originates here, and flows south and "off" the edge of the caldera at Mesa Falls. We visited the upper falls, which has an old inn/lodge and pathways with viewpoints. Since we visited in the morning, we caught the sun in the right place to see the rainbow!