GCT '09

Built to prevent water overflowing the dam, the concrete-lined open channels about 650 ft long, 150 ft wide, and 170 ft deep sit on each canyon wall. Water is discharged from the spillways into the outer diversion tunnels through inclined shafts 50 ft in diameter and 600 ft long. The discharge is controlled by four rum gates on each spillway crest. Maximum water velocity in the spillway tunnels is about 120 miles per hour with a volume per second equal that of Niagara Falls.