Ooh, that's an interesting idea! I'll have to try that at some point; I can imagine that it allows the rivers to be more varied in size, instead of just tapering off uniformly as they do on this map. But wouldn't you have more flexibility if instead of erasing them from the land layer, you created an alpha mask for it and painted the rivers in as black? That way, you wouldn't have to worry about losing the land texture if you decided you were unsatisfied with the rivers you made.