A strong dam depends not just on the visible concrete but on the unseen structure below river level. During construction, engineers drilling for bedrock encountered deep, unstable deposits of glacial silt, loose gravel, and sand. Water seeped through and undermined diversion dams built to allow excavation, so crews inserted a network of refrigeration tubes into the till and created a giant ice plug that froze the gravel in place so excavation could continue. Completed in 1961, the 300-foot-high (90-meter) dam is set into bedrock and extends almost an equal distance above the river bed and down into the thick layer of gravel and sand between river and rock. Glaciation and other processes that shaped this gorge over the past million years created expensive challenges for dam-building.