I've created this massive landscape in Gaea 

Gaea allows to simulate how water flows, erosion, temperature & time affect rock formations & growth on them. 

I've exported all this data from Gaea & will use it as masks in Unreal Engine - this masks guide me there I can put trees, snow, soil, different rock materials and so on.

This data doesn't do my job for me, but logic behind these simulations, even though I control how they look, helps to ground my creative choices in true-to-life principles (trees seek water etc.)