 When the cofferdams were in place and the construction site was drained of water , excavation for the dam foundation began . For the dam to rest on solid rock , it was necessary to remove accumulated erosion soils and other loose materials in the riverbed until sound bedrock was reached . Work on the foundation excavations was completed in June 1933 . During this excavation , approximately 1 @,@ 500 @,@ 000 cu yd ( 1 @,@ 100 @,@ 000 m3 ) of material was removed . Since the dam was an arch @-@ gravity type , the side @-@ walls of the canyon would bear the force of the <unk> lake . Therefore , the side @-@ walls were excavated too , to reach virgin rock as weathered rock might provide pathways for water seepage . 
