Three basic methods:
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Buy fluid ninja (or another similar plugin that simulates fluids)
FluidNinja LIVE - UseCase 7 LEVEL WALKTHROUGH - Waterfalls, Pools, Fountains - YouTube -
Use a static mesh and create a panning material similarly to how you would make a river
Forest Stream Waterfall - Foam and Finishing Touches - YouTube -
Use a particle system (like in niagara)
Waterfall Effect in UE4 Niagara Tutorial - YouTube
Hopefully that helps you get started!