Ben Ehrenreich is the author, most recently, of “Desert Notebooks: A Road Map for the End of Time.” THE AMUR RIVER Between Russia and China By Colin Thubron 291 pp. Harper. $27.99.

The Amur River, writes Colin Thubron, is “one of the most formidable rivers on earth.” It is also one of the most mysterious and elusive: Estimates of its length vary wildly, as do ideas about ...