In 1956, work began on the two diversion tunnels that would carry the Colorado River around the dam site during construction. Each of the tunnels was in diameter, with a combined capacity of ; the right-side tunnel was long and the left . The right tunnel would be used for carrying the Colorado's normal flow around the dam site, while the left tunnel, above the water, would only be used during floods. The lower reaches of the tunnels would later be used to form the lower ends of the dam's spillways. About of material would have to be excavated from the diversion tunnels.