Garrison Dam’s spillway has been used exactly once. That was 2011, when record Missouri River runoff forced the Corps to open it for the first time since the dam went into service in 1953. The water dislodged one of 27 manhole covers on the spillway’s drainage system and damaged the subdrain beneath the chute slab. The analysis that followed found something worse: a larger flood could hydraulically jack the slab off its foundation and fail the spillway outright. Fixing that is now a $1.8 billion project, the largest dam safety modification in the history of the U.S. Army Corps of Engineers.
Project Scope
Garrison is an 11,300-foot earthfill embankment standing 210 feet at maximum height, impounding Lake Sakakawea at 23.8 million acre-feet. The Dam Safety Modification Study completed in June 2023 set the scope. The spillway drainage system gets replaced in full. A reinforced concrete overlay goes across the chute and stilling basin. Crews will armor behind the chute walls, raise the abutment monoliths against overtopping, install a new deflector beam, modify the trunnion hubs on the gates and upgrade the gate de-icing system.
Design capacity rises from 827,000 cubic feet per second to 1.15 million. Roughly $45 million has been secured for preconstruction engineering and design, and construction is scheduled to begin in 2030. Ahead of the main work, the Corps awarded a $23.8 million dredging and riprap contract to HSG Park Joint Venture 2 in June 2026, running through April 2028.
Why It Matters
Dam safety work rarely gets the coverage a new bridge does, and the reason this one deserves it is the failure mode. The 2011 event wasn’t a near-miss in the ordinary sense. It was a single use of a spillway that revealed the structure couldn’t survive being used harder, on a dam holding back the second-largest reservoir by surface area in the country.
The Corps has hundreds of high-hazard dams built in the same postwar decade under the same design assumptions. Garrison is the first at this scale to get a full rebuild of its release works, and the contracting model is worth watching: a seven-year runway from study to construction start, with advance packages let separately to take risk out of the main job.
Project Team & Details
| Owner / Client | U.S. Army Corps of Engineers, Omaha District |
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| Major Subcontractors | HSG Park Joint Venture 2, LLC (Dredging and Riprap) |
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| Status | Planned |
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| Delivery Method | Design-Bid-Build |
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| Funding Source | Public (Federal) |
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