Nobody knew what the Colorado did once it entered the canyon country. There were stories of falls the height of Niagara and of stretches where the river went underground. Powell had a few instruments, some flour and bacon, and a conviction that a river could be read like rock if a man paid attention.

Two weeks in, one boat struck a rapid the party named Disaster Falls and broke apart, and a third of the provisions went with it. The rest spoiled slowly through the summer. The men ran the rapids by day and dried their clothes by fire at night. Powell climbed the canyon walls one-handed to take his readings, and on at least one ledge got stuck and was hauled up by a pair of long johns knotted into a rope.

By late August they were deep in the Grand Canyon, eating moldy flour and facing a stretch of white water worse than anything behind them. Three men decided they had had enough. They climbed out at a place now called Separation Rapid to walk for the settlements. Powell and the other five ran it. Two days later, on the thirtieth of August, the river let them out into open country. The three who walked were never seen again.

Powell went back and ran the whole thing a second time with better boats and more men. He went on to found the Geological Survey and to argue, decades before anyone wished to hear it, that the arid West would have to be settled around its water rather than its wishes. He was ignored. He was also correct.

Every venture has its Separation Rapid, a point where the map runs out and the sensible thing is to climb the wall. Sometimes climbing is right. Sometimes the river is the only way through. Read the water, decide, and do not look back at the ledge.