Complex sentence: Since the development of the Colorado River Compact, California has been using the surplus water that has been left over from other states; however, with increasing population growth in the Southwest there is concern that this surplus soon will not exist for California's use.
Split into simpler sentences: Since the development of the Colorado River Compact, California has been using the surplus water that has been left over from other states. With increasing population growth in the Southwest there is concern that this surplus soon will not exist for California's use.