I've tried create feeling of my local parks in Cornwall, UK (not exact recreation).
Dense, bushy forest. River is more mountain alike.
Instead modelling I tried use different asset from packs and concentrate more on composition and design.
I haven't done too much optimization, but still with some basic techniques I managed run it around 30fps, with occasional hick ups (probably stream levels would help with that...)
In this project I used:
Megascans and Quixel Mixer
City Park Pack (Provided for free on Epic's Marketplace)
Branching River Tool (From Marketplace)
Volumetric Clouds (Experimental UE4 Plugin)
Ultra Dynamic Sky (From Marketplace)
Chameleon PostProcess for slight colour touch (From Marketplace)
Meadow - Environment Set (From Marketplace)

Please watch video in 4K for good quality.