The idea behind a spillway is simple: Prevent water from rising too high in a reservoir. Much like the overflow tube in your sink or bathtub, the spillway of a dam allows water that has reached a critical height to be diverted into an enormous drain to be released into the river below. A Bell-Mouth Spillway, so named due to its shape, is unique in that it is placed within the reservoir itself. In dry periods, it merely looks like a ring-shaped column of cement protruding out of the water. But all of this changes when the reservoir starts to fill.
Once water levels rise high enough to reach its top, this type of spillway actually disappears. The water flowing over the edge is the only indication of its presence. It becomes a black hole in the midst of tranquil waters. If this occurs near a roadway, you will often see onlookers crowding near to see it in action. But the inner workings of this type of spillway are hidden deep within the black depths of the dam. You can’t see where it goes, but you know it’s deep.
Another part of the horror of this type of spillway comes from its uncontrolled operation. There is no mechanism preventing it from starting operation. Once water levels reach the top of this drain, the water goes in. No technician turns it on, no gate prevents objects from flowing in. There’s no intelligence at work here; it is simply a giant drain in a lake.
At least one person has been pulled into a Bell-Mouth Spillway and drowned. In 1997, a woman succumbed to the so-called “Glory Hole” in Lake Berryessa, in Napa, California. One account says she was fully clothed and distraught when she entered the lake. She then either swam or was pulled toward the opening of the spillway. As she was swept up and over, she grabbed onto the concrete ledge and clung there for twenty minutes, slowly losing her strength as the power of the water flowed around her. Onlookers could only watch until, finally, she disappeared inside. Her body was found in the river below, having made the journey completely under the dam and out the spillway’s exit.