Complex sentence: Unlike the Don to the east, the Humber remained relatively free from industrialization during the grown of Toronto in the 1950s, mainly because it is much flatter and does not provide a large river valley to build in.
Split into simpler sentences: Unlike the Don to the east, the Humber remained relativily free from industrialization during the grown of Toronto in the 1950s. This is due largely to it being much flatter and not providing a large river valley to build in.