Take it a step further NHawk, try this:
Set a river tile, any will do so go with River 1.
Then click the Manual button under the list of SubTypes.
Now in the upper left quadrant of the dialog window, click radio button 3.
Click OK.
Originally I thought the river textures were different from the Grass textures. After all, they're a different size and the SubType radio buttons become active when a River tile is selected. Wish I had the ability to make good tiles with better transitions but I'll have to wait for you guys.
Let's supposed you decided to go with the old way of doing rivers by setting water tiles at elvation 0. Now you have 16 river tiles counting SubTypes, plus the quarter sections, to play with for realistic beaches and river fronts, and you might still have some left over for other things. I think the limiting factor would be the ovly[_a]#### files.