I think what you want is a little program called Wilbur. It's a free utility written by Waldronate that does erosion modeling on heightmaps. It's a little idiosyncratic and can be difficult to use at first, but if you follow the Fun With Wilbur tutorials at the bottom of that page, you should be up and running with it relatively quickly. Better still, if you have questions, just post a thread here with Wilbur in the title, and Joe will almost certainly show up to answer.