Complex sentence: Concrete pouring began on January 19,1938 and the last bucket was emptied on December 21,1938; water storage began in April 2,1939 when the outlet tunnels were closed.
Split into simpler sentences: Concrete pouring began on January 19,1938, and the last bucket was emptied on December 21,1938. Water storage began on April 2,1939, when the outlet tunnels were closed.