Thanks guys. The glacier is 1k high and quite a distance away (around 100k I think). I'm doing a series of things, but needed to find out whether an ice age sheet would be visible from a certain area. Not, I'm afraid, so this is for fun. As soon as the sheet of max 1k high is further than 300k away, it's pretty invisible. So there goes my 'romantic' idea to visualize what I wanted.
The rivers I made beforehand (Daniil's shader indeed + ortho render rivers illuminated on a black surface), as that's lower on MB's than keeping an erosion file, and I could work on them a litttle in PS, and make a soft 'off river' map for the rises. Distinction between gravel banks and rivers is by color adjust from the river map.