The two spillway tunnels are excavated through the canyon walls on each side of the dam. Twin radial gates, each wide and high, control the flow of water into the spillways. Together, the spillways can pass up to . The tunnels required of excavation and another of concrete lining. The circular, concrete-lined spillway tunnels plunge at a 55-degree angle, reducing in diameter from , until they intersect with the old river diversion tunnels at sharp elbow joints before returning to the Colorado River. This was done as a cost-saving measure, but resulted in the destruction of both spillways during the 1983 flood releases. The repairs, in which air slots were installed to prevent cavitation shock waves, cost about $15 million.