General:
* Rushing
* Gushing
* Turbulent
* Swift
* Rapid
* Torrential
More specific:
* Cascading (for water falling over a steep drop)
* Raging (for water flowing with great force)
* Whirlpooling (for water flowing in a circular motion)
* Foaming (for water churning and creating bubbles)
* Churning (for water moving with great force and creating turbulence)
* Roaring (for water making a loud sound as it flows)
Figurative:
* Furious
* Unstoppable
* Impetuous
* Relentless
The best word will depend on the specific context and the desired effect. For example, "rushing" might be appropriate for a gentle stream, while "torrential" would be more fitting for a raging river.