Well I don’t do much landscaping, but I bet the topology of the mesh are hardly ever quads, most likely it’s all very dense triangles where you can sculpt indentations to the mesh and form riverbanks. I would approach it just forgetting about quads and just get a bunch of polygons in there so you have enough geometry to form all the ups and downs and irregularities a landmass would have. There’s a good course you can get about it from the folks of Cg Boost I think:
https://www.youtube.com/watch?v=yBP5x87HNXE
