wow, sorry I didn't answer this earlier. Wilbur is a piece of software that helps you shape a landmass in a geomorphical manner. You can erode your landscape with several different methods, you can fill basins, add grain, and best of all, you can make it slice in rivers exactly where nature would. It's a fantastic piece of technology and it's absolutely free. The website is a bit... shall we say oldfashioned? But it sports several tutorials and of course the download link. Current Wilbur Version

Have a go at it and let us know what you think!