Going the direction of using various colored tiles might seem like a good idea (as I just mentioned), but after thinking about it for a few minutes it would be easier to just cut a felt river and drop it on a homogeiously colored board than to fit border felt along the sides. :roll:
Yet another take on rivers though would be to have carved-out river sections that could be fitted together. The issue with that though is that if you used 12" square sections it wouldn't "come off" as very realistic. However, getting back to my 24" sections, I could "wind" them much more randomly before they hit a "border crossing". If a person worked at it, they might be able to use the 12" for it though.