Why Does Water Released from a Dam Erode the Streambed

Why does water released from a dam erode the streambed so much more than it would have before the dam existed?

A) It has a greatly increased velocity.
B) It has more stream power than it would have had before the dam.
C) The dry stream bed below the dam weakens while it is left dry.
D) The stream power now exceeds the sediment load by a great amount.