Complex sentence: The huge volume of water that flows down the Damodar and its tributaries during the monsoons used to be a fury in the upper reaches of the valley but in the lower valley it used to overflow its banks and flood large areas.
Split into simpler sentences: The huge volume of water that flows down the Damodar and its tributaries during the monsoons used to be a fury in the upper reaches of the valley. In the lower valley it used to overflow its banks and flood large areas.