The computer hums while processing data. Eventually Gardiner pulls up a graph profiling the river's bed. It looks like a U - as smooth as a mountain valley carved by a glacier. The current just beneath the surface is traveling at 30 miles per hour, and the channel is 640 feet deep. That's the deepest point measured on a river in the world," Gardiner says. "There's no question about that."
Shelton is peering over Gardiner's shoulder, shaking his head and deciphering blue and red lines on the computer screen that represent water movement and velocity.
"Just like we thought," he says. "Fabulous stuff." He nudges a moth off the screen and points to a place in the riverbed where a long blue line indicates the current dropping vertically from a ledge into the canyon's trough.
It's an underwater waterfall," he says, slapping Gardiner's shoulder. It's falling at 40 feet per second. Upstream of the waterfall is an eddy, the water relatively still. This point is likely habitat for the blind cichlid: calm pockets where sheering currents have trapped the fish at great depths. Deep-river specimens, like the one found today, surface only when the river surges and flushes individuals into the harsh environment of the main flow. In terms of Stiassny's hypothesis, the finding suggests that the Congo's currents partition habitat from side to side and from top to bottom - just like a mountain range.
"It shows water can be an evolutionary barrier, even for fish," Gardiner says.
