I have a slight idea on what I wanted to do to solve this; this is a 10% - 40% gradient (give or take a few % at the minimum), and the canyon the Rio Grande (on the right) is about the height I want it. To actually make the place for the river, in photoshop I used a 3-4 point brush of 7-8% black to trace the riverbed, before the gaussian blur, and that came out nice. But the mountain range up near Los Alamos itself is way, way too high still. I aimed to lasso an area in the heightmap editor and then try to blanket lower that area by a low intensity, then lasso a slightly smaller area, lower it by a low intensity, rinse, wash, repeat, and then a blanket blur of that area. I'd do something much less convoluted in photoshop, but where the boundary of the lasso selection and the unselected section always has an abrupt end that I can never get to blur to be a nice transition. I'm aware the heightmap editor has the erosion function, but trying to toy around with it, every time I ran it it'd crash the GECK, so I assume it either doesn't work or I'm making it do things it doesn't want to. Feel like a needy child at this point, but I'm not positive I'll figure this out on my own.