The kayakers battled whitewater at the river’s head for 130 kilometers over five days, taking depth readings of the river bottom all the while. The data was astounding. “We couldn’t believe it,” says Stiassny. “It’s almost like you’re boating over a mountain range. There are huge peaks and enormous troughs. This is the deepest river in the world, there’s no question about it.” In one stretch, the echo sounder registered depths of 220 meters—much deeper than the previous record-holder, the Yangtze River.
The Lower Congo’s extraordinary riverbed topography results its extreme rapids, whirlpools, and underwater waterfalls. Stiassny and Shelly’s data are revealing that these water movements act like brick walls to fish: they separate populations so they can’t breed together.