The science of tree rings--"dendroclimatology"--had not been developed when John Wesley Powell made his epic voyages down the Colorado River in 1869, 1871, and 1872. Nevertheless he observed that the rising and falling of the river differed over the years and came to understand the important role these variations played in the lives of people trying to live in the West.

While Powell was braving the Colorado River's rapids, a tree in southwestern Colorado was putting on rings. In 1869 it was a modest ring. In 1871, the year Powell returned to begin his second trip, the ring was remarkably...