The large spillway tunnels have been used only twice , for testing in 1941 and because of flooding in 1983 . During both times , when inspecting the tunnels after the spillways was used , engineers found major damage to the concrete linings and underlying rock . The 1941 damage was attributed to a slight misalignment of the tunnel invert ( or base ) , which caused cavitation , a phenomenon in fast @-@ flowing liquids in which vapor bubbles collapse with explosive force . In response to this finding , the tunnels was patched with special heavy @-@ duty concrete and the surface of the concrete was polished mirror @-@ smooth . The spillways was modified in 1947 by adding flip buckets , which both slow the water and decrease the spillway 's effective capacity , in an attempt to eliminate conditions thought to have contributed to the 1941 damage . The 1983 damage , also due to cavitation , led to the installation of aerators in the spillways . Tests at Grand Coulee Dam showed that the technique worked , in principle .