Complex sentence: The immense amount of rain the Black Hills received during the thunderstorms ranged from 4 to 15 inches, causing Rapid Creek and surrounding creeks to overflow and creating massive amounts of runoff that resulted in flood waters.
Split into simpler sentences: The immense amount of rain the Black Hills received during the thunderstorms ranged from 4 to 15 inches. Causing Rapid Creek and surrounding creeks to overflow, along with massive amounts of runoff resulting in flood waters.