Complex sentence: The most significant flood of the first half of the 20th century was the St. Patrick's Day Flood of March 1936, which also reached Pittsburgh and became known as the Great Pittsburgh Flood of 1936.
Split into simpler sentences: The most significant flood of the first half of the 20th century was the St. Patrick's Day Flood of March 1936. It also reached Pittsburgh, where it was known as the Great Pittsburgh Flood of 1936.