On May 24, 1869, four wooden boats pushed off from Green River, Wyoming, into water that flowed toward a blank spot on the map. Ten men were aboard. Their leader was a thirty five year old geology professor with one arm. John Wesley Powell had lost the other one to a musket ball at the Battle of Shiloh, and he had gone back to the war after it healed.

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

Two weeks in, one boat hit a rapid the men named Disaster Falls and broke apart. A third of the food went with it. The rest spoiled slowly through the summer. The men ran rapids by day and dried their clothes by fire at night, and Powell climbed the canyon walls one handed to take readings, more than once getting stuck on a ledge and being pulled up with a pair of long johns for a rope.

By late August they were deep in the Grand Canyon, eating moldy flour and facing a stretch of whitewater worse than anything behind them. Three men decided they had had enough. They climbed out at a place now called Separation Rapid to walk to the settlements. Powell and the remaining five ran the rapid instead. Two days later, on August 30, the river spat them out into open country. The three who walked were never seen again.

Powell went back and ran it again two years later with better boats and more men. He went on to found the U.S. Geological Survey and to argue, decades before anyone wanted to hear it, that the arid West would have to be settled around its water and not around its wishes. He was ignored. He was also right.

Every venture has a 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. Then decide, and do not look back at the ledge.