How Does Glory Hole Work?
Lake Berryessa is a man-made lake (reservoir) in Napa County that was created in 1958 with the construction of Monticello Dam. It is the seventh-largest lake in California. During wet winters the when the lake hits capacity (defined as a lake level above 440 feet msl), water starts flowing over this circular spillway popularly known as the Glory Hole - more formally known as a Morning Glory Spillway - into Putah Creek on its way to the City of Winters.
The spillway is 72 feet wide at the top and curves down to about 28 feet wide at 20 feet below the spillway lip. The spillway can drain 48,400 cubic feet per second (96,000 acre-feet per day) in the unlikely event that the lake level would rise to 15.5 feet above the level of the funnel - which is almost the height of Monticello Dam (456 feet above mean sea level - msl).
48,400 cubic feet per second in a full 28 foot diameter circular pipe means the water would be flowing at 53 miles per hour.
If the lake ever reached 455.5 feet msl the system is designed so that water would first go over Highway 128 before it overtopped the dam. The highest level the lake ever reached was 446.7 feet msl in 1983.
During dry years skateboarders have used the Glory Hole output tunnel for doing tricks.
