The artist utilized Unreal's Fluid Flux plug-in to create the river.
Vegetation Technical Artist at Ubisoft Paris Michael Gerard has demonstrated Jungle River, a picturesque jungle scene inspired by Horizon Forbidden West. According to the artist, the idea to create such a scene first appeared after the release of the first Horizon game, however, the project was postponed after the announcement of HFW because the artist knew that the new game's environments would be even more majestic.
The project's description states that the scene was made using 3ds Max, Substance 3D Designer, SpeedTree, Megascans assets, and Unreal Engine 5. Additionally, the artist utilized Unreal's Fluid Flux plug-in to create the river itself.