At Lake Berryessa in Northern California stands an engineering wonder that simultaneously inspires awe and unease—the Morning Glory Spillway, colloquially known as the “Glory Hole.” This remarkable structure, resembling a giant bathtub drain emerging from the lake’s surface, holds the distinction of being the largest drain in the world and serves as the crucial overflow mechanism for the Monticello Dam.
Rising 72 feet from the lake bed, this massive concrete funnel with its perfect circular rim spans an impressive 72 feet in diameter. When the lake’s water level reaches its capacity, this colossal drain awakens, creating a mesmerizing vortex that channels excess water through its gaping mouth. The water spirals dramatically downward through the vertical shaft before making a 90-degree turn into a 700-foot long tunnel that eventually releases into Putah Creek below the dam.
The physics behind this marvel are both simple and complex. Operating purely through gravity, the Glory Hole can process an astounding 48,000 cubic feet of water per second when flowing at full capacity. That’s equivalent to draining about 1.6 Olympic-sized swimming pools every second—a volume that transforms the placid lake surface into a swirling spectacle of hydraulic power.
Constructed in the 1950s as part of the Solano Project, the Morning Glory Spillway represents a brilliant solution to water management challenges in California’s drought-prone climate. The Monticello Dam itself, standing 304 feet tall, creates Lake Berryessa—a reservoir holding over 1.6 million acre-feet of water that provides irrigation, drinking water, and flood control for the surrounding regions. The spillway ensures that even during the heaviest rainfall, water levels remain safely below the dam’s crest.
What makes the Glory Hole particularly fascinating is its relative rarity of operation. Due to California’s precipitation patterns and water management practices, years can pass without the lake reaching the critical 440-foot elevation that triggers the spillway. When drought conditions persist, as they frequently do in California, the giant drain remains dormant—a concrete sentinel waiting for the waters to rise. This intermittent activation transforms its functioning into a notable event, drawing spectators from miles around when conditions align.
The visual impact of seeing the Glory Hole in action is unforgettable. From elevated vantage points, observers watch as the smooth lake surface develops a visible current that accelerates toward the spillway’s mouth. The water forms a perfect circular waterfall, dropping into the vertical shaft with a thunderous roar that echoes across the lake. The resulting vortex creates mesmerizing patterns of swirling water that demonstrate fluid dynamics on a massive scale.
Despite protective barriers and warning signs, the Glory Hole has developed a somewhat ominous reputation over the decades. Its powerful suction and the sheer scale of water movement have made it both a tourist attraction and a reminder of nature’s raw power when channeled through human engineering. Local authorities maintain strict security measures, especially when the spillway is active, ensuring that curious onlookers maintain a safe distance.
Beyond its practical function, the Morning Glory Spillway has become a cultural touchstone—featured in photography collections, engineering textbooks, and even as a backdrop in films. Its perfect circular form cutting through the lake’s surface creates a surreal juxtaposition against the natural landscape that surrounds it. Photographers particularly prize capturing the spillway during sunset, when light plays across the moving water, creating ephemeral rainbows in the mist that rises from its depths.
