Prado Dam Spillway Modification: A $1.13B Labyrinth Rebuild in Corona

Prado Dam has been holding back the Santa Ana River since 1941, and the U.S. Army Corps of Engineers is about to take its spillway apart.

The Los Angeles District awarded a $1.13 billion contract to Barnard Construction Company in March 2026 to rebuild the spillway at the dry dam above Corona. Preconstruction and design are underway now. Significant construction isn’t scheduled to start until spring 2027, and the work runs through 2033.

Project Scope

The centerpiece is a new labyrinth-style control structure. Labyrinth geometry folds the crest back and forth so the weir gets far more effective length than the width of the chute it sits in, which is the whole point here: the basin is hemmed in by urbanization and there is nowhere to widen. Every chute wall and slab on the existing spillway comes out and gets rebuilt, tied back into the earthen embankments at both ends. A second phase raises the spillway crest 20 feet to match the main embankment raise completed in 2005. The Corps folded both phases into the single Barnard contract rather than bidding them separately.

The dam stays operational the entire time. That is a staged-diversion problem of the same class as Whittier Narrows and B.F. Sisk, both of which are also under seismic and hydrologic retrofit.

Why It Matters

Prado protects roughly 1.4 million people and more than $200 billion in property between the dam and the Pacific. It is a dry dam, so the basin normally holds very little water and the structure spends most of its life doing nothing visible. That is exactly why deferred spillway work at these facilities tends to sit until a dam safety study forces the issue, which is what happened here.

Labyrinth spillways at this scale are still uncommon in the United States. Contractors bidding federal dam safety work over the next decade should expect to see more of them, because the same constraint applies almost everywhere: the old chutes were sized for old hydrology, and there is now a subdivision where the widening would have gone.

Project Team & Details