By April 1959, excavation of the diversion tunnel had been completed and concrete lining was finished on August 17. Work on a pair of earthen cofferdams above and below the dam site commenced when the tunnel was ready and the river was channeled around the dam site on November 19 with the completion of the upper cofferdam. Keyway (foundation) excavations for the dam on the right abutment and construction of the spillway inlet works in the left abutment was begun in September, and all preliminary canyon wall structures were complete by early 1960. The lower cofferdam was finished in February, allowing workers to pump water out of the space between the two barriers. The silt and sediment that comprised the riverbed had to be removed in order to reach solid rock where foundations could be drilled; this was completed in August 1960, allowing work on the main dam foundations to begin.