Along the Magdalena River, one of Colombia’s primary arteries, fishermen move with wary precision. From the silty, tea-colored current, hippopotamuses can surface without warning, closing the distance to a vessel in a single heartbeat.

“These hippos are a true nightmare,” said fisherman Wilinton Sánchez about the semi-aquatic animals capable of charging at speeds of up to 8 kilometers (5 miles) per hour in water and 30 kilometers (18 miles) on land. “We were out Saturday when one lunged … reared up and swung its jaws wide. If it ever gets hold of you, it’ll tear you to pieces.”

The animals also draw fascination.

Several afternoons a week, boats filled with Colombian and foreign tourists arrive to scan the shoreline waiting for signs in the murky water.…