Yes, you can do it in the Terrain Editor, but it takes a bit of practice. The Photoshop method that wabe is describing is also helpful, but it won't let you make an undercut bank, like I think you are describing for the erosion-steeper slope area. It can get you close to you ideal layout, and then you can use the Terrain Editor to tweak it further. If you don't have Photoshop, it can be Corel Paint, Painter, Paint Pro, or any of those things. Terragen is a free program that will also let you do this - needs skill about midway between that of wabe's method and skill at the Terrain Editor. Unfortunately, when I need to do really precise terrain work with stream banks, I use Truespace - a standard modelling program. But, a lot of the reason for this is to keep the polygon count of the mesh within reasonable limits. Can you post a sketch of what you want? Do you have some drawing tool that you can use to create and post a sketch. Wabe and I might be able to more help to you that way.