WITH A RISING SENSE OF anticipation, we moved between cantering and trotting, making what felt like fantastic time. The last bits of scattered forest gave way to grasslands, alongside which a river in flood raged. I watched it while clinging tight to the saddle, seeing bits of wood wash past, a tree trunk stripped till almost bare. The occasional rumbling thud rang out, vibrating through the earth when loosened boulders crashed along, unseen beneath the water.

We followed the ...