Complex sentence: Membership peaked at over 180 families soon after the move to the new building, but during the 1970s Niagara Falls had entered a long economic downturn: plants run by major employers closed, and the city's population dropped by over 40 percent.
Split into simpler sentences: Membership peaked at over 180 families soon after the move to the new building. During the 1970s Niagara Falls had entered a long economic downturn: plants run by major employers closed, and the city's population dropped by over 40 percent.