Lava Falls!

Lower Grand Canyon, Arizona, USA. As part of a US Geological Survey expedition in May, 1976. The blue flag on the boat has the USGS logo. So I got to do as part of my job what people _pay_ to do for fun ;)
Lava Falls is the largest rapid in the Grand Canyon. It occurs toward the end of a river trip, in the lower canyon, so you have the feeling it's "hanging over your head" most of the trip!