In addition to flooding and disaster declarations, the recent bout of storms has revived a singular phenomena in Napa County, the Lake Berryessa “water hole.”

Last seen in early 2017, this strangest of sights is the lake’s 72-foot-wide, funnel-shaped concrete spillway in action.

In rain-heavy years, when there’s danger of a reservoir overflow and subsequent flooding, the spillway sends excess water plunging down a 200-foot drop that provides drainage relief.

When the reservoir level is lower, the drain is plainly visible for exactly what it is—a large concrete structure sitting placidly on the south side of the lake.

But when the reservoir surges over the 440-foot level, the results are otherworldly—and potentially alarming for observers who don’t know what they’re seeing.

According to the Solano County Water Agency, Lake Berryessa sits at 443 feet as of March 1, up from 428 feet on February 1.

Popular Mechanics reports that the spillway can drain water at a rate of up to 48,400 cubic feet per second, which accounts for some of the hypnotic qualities of its action.

According to the U.S. Bureau of Reclamation, “Lake Berryessa [...] formed when the Bureau of Reclamation built Monticello Dam in 1957. [...] Lake Berryessa has a storage capacity of 1,602,000 acre-feet.”

Note that the design for this kind of spillway is known as a “morning glory spillway,” so named after morning glory flowers.

This means that, inevitably, many people end up referring to it as a “glory hole” instead. Unfortunate.

Here are some scenes of the spillway in action:

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