Why Glen Canyon Dam started spitting boulders out of its spillway in 1983

In June 1983, record snowmelt packed Lake Powell almost to the brim, and Glen Canyon Dam's spillway tunnels started spitting out boulders. Water racing through at about 120 miles an hour was tearing the concrete apart through cavitation: vapor bubbles imploding like millions of hammer blows. Closing the gates risked the lake rising over them; opening them wider meant more damage. So engineers made the dam temporarily taller. The spillway gates were the lowest point on top of the dam, and crews bolted four feet of plywood onto them in about 22 hours, later swapping in 8-foot steel panels. The lake peaked just inches below the top. The permanent fix was air: slots that cushion the bubbles. Sources: • McCormick 2021 — http://www.riversimulator.org/Resources/USBR/DamSafety/GlenCanyonDamRepairsSpillwayTunnels1983McCormick2021.pdf • Hannon 1999 — http://www.riversimulator.org/Resources/GCDAMP/GCDAMPchronicle/1983FloodGCDhannon.pdf • ASDSO — https://damfailures.org/case-study/glen-canyon-dam-arizona-1983