A spillway is a channel or passage built into a dam that allows excess water to flow safely out of a reservoir when it gets too full. Think of it like an overflow drain in a bathtub: when water rises too high, it needs somewhere to go, or the tub will overflow and flood your bathroom.

Dams hold back enormous amounts of water to create reservoirs for drinking water, irrigation, and hydroelectric power. But during heavy rains or spring snowmelt, more water flows into the reservoir than usual. Without a spillway, the water would keep rising until it spilled over the top of the dam or, worse, damaged or destroyed the dam completely. A failing dam could unleash a catastrophic flood downstream, destroying towns and endangering lives.

Engineers design spillways to handle these emergencies. Some spillways are gentle concrete channels that guide water down the side of a dam. Others are dramatic: the spillway at California's Oroville Dam looks like a giant concrete ski jump where water rockets down at incredible speeds before rejoining the river below.

When you hear that a dam's spillway has opened, it means the reservoir is full and engineers are deliberately releasing water to keep everything safe.