Complex sentence: The high, steep hills of the narrow Conemaugh Valley and the Allegheny Mountains range to the east kept development close to the riverfront areas, and subjected the valley to large amounts of runoff from rain and snowfall.
Split into simpler sentences: The high, steep hills of the narrow Conemaugh Valley and the Allegheny Mountains range to the east kept development close to the riverfront areas. The valley had large amounts of runoff from rain and snowfall.