Complex sentence: The eastern two thirds of Oregon have cold, snowy winters and very dry summers; much of it is semiarid to arid like the rest of the Great Basin, though the Blue Mountains are wet enough to support extensive forests.
Split into simpler sentences: The eastern two thirds of Oregon have cold, snowy winters and very dry summers. Much of the east is semiarid to arid like the rest of the Great Basin, though the Blue Mountains are wet enough to support extensive forests.