Material dug out of the tunnels and the dam abutments on the canyon walls was used to build the two cofferdams to divert the Colorado River, which were complete in February 1960. The upper cofferdam was high, and it alone could store several million acre-feet of water to protect the dam site from flooding in the event that inflows exceeded the capacity of the diversion tunnels. On February 11, 1959, the right diversion tunnel was completed and began to carry the flow of the Colorado. The left tunnel was finished over three months later on May 19, 1959, slightly behind schedule.