Consider a couple of scenarios for big trouble at Oroville Dam: First: The facility’s main concrete spillway suffers serious damage, resulting in erosion of the rock beneath it — and potentially threatening the safety of the dam itself. Second: Water fills Lake Oroville, the gigantic reservoir behind the dam, and begins surging down a steep unpaved hillside that’s meant to serve as an emergency spillway. The slope suffers serious erosion, again potentially threatening the dam’s safety.