Thanks for the very useful info!

Your suggestion to add known high and low spots to the map is especially helpful, and I have already implemented it as I begin the process of erosion yet again. I'm finding that it's generally important to get the map right before starting erosion, as it's easier to make changes in the early design and smooth them over with erosion than to tweak the already-eroded map, so I'm going back to my design stage. But thanks for the info about noise. Wilbur has a lot of functions that I haven't played with yet, so it's nice to have learned about another one.