For National Park Service fisheries biologist Jeff Arnold, it was a moment he'd been dreading. Bare-legged in sandals, he was pulling in a net in a shallow backwater of the lower Colorado River last week, when he spotted three young fish that didn't belong there. “Give me a call when you get this!” he messaged a colleague, snapping photos.

Minutes later, the park service confirmed their worst fear: smallmouth bass had in fact been found and were likely reproducing in the Colorado River below Glen Canyon Dam.