The giant hole this lake is supposed to have

The giant hole in California's Lake Berryessa belongs there. It's the entrance to Monticello Dam's Morning Glory Spillway, a funnel built to carry excess lake water into Putah Creek downstream. At 72 feet across, the opening looks like something has punched through the surface. But the lake isn't collapsing, and the water isn't disappearing underground for good. It's taking a deliberately built shortcut around the dam. What looks like a problem is part of the solution to keeping a reservoir from getting too full.

The trigger is the height of the water. When the lake rises above the spillway's rim, at roughly 440 feet of elevation, water pours over the circular edge. That number describes the lake surface's elevation, not a 440-foot depth inside the hole. The useful household comparison isn't the drain at the bottom of a bathtub. It's the overflow opening near the top of a sink: water has to reach it before it can escape through it. Below the rim, the spillway doesn't drain the reservoir through its mouth. Above it, gravity gets to work.

The visible funnel is only the entrance. Water drops about 200 feet down a narrowing shaft, which reaches about 28 feet across at the bottom. Then its route makes a 90-degree turn and continues through a tunnel toward the downstream side of the dam. It emerges into Putah Creek, back in an open watercourse. The broad, curved mouth helps water enter smoothly, so the surface can look relatively orderly even while the flow below is anything but gentle. The quiet-looking edge is the beginning of a substantial fall.

Why build a vertical funnel rather than a more familiar open spillway? Space and rock. The site offered limited room and a suitable rock foundation, making this drop-inlet design a practical fit. The dam holds water back; the spillway gives surplus water a separate way past it. Engineers have used this bell-mouth arrangement at other dams, too. Berryessa's version is visually strange, but the underlying idea isn't an engineering one-off. A wide entrance collects the overflow and directs it into a much more compact passage below.

The spillway can carry up to 48,000 cubic feet of water per second. That capacity belongs to an engineered system with limits, not an endlessly hungry hole. In morning-glory spillways, sufficiently high flows can shift the point controlling discharge from the rim to the narrower throat. Tunnel spillways can also trap debris, and removing a blockage may have to wait until the flow subsides. The size of the mouth isn't the whole story. What happens farther down matters, too.

The spillway doesn't run every year. Its performance depends on the reservoir rising high enough, with incoming water from the watershed helping determine how long spilling continues. That's why the same structure can sit exposed above the lake at one time and become a circular waterfall at another. Nothing has opened beneath the lake. The lake has climbed above an opening built to wait for it.