@ Gothmog: Go to the channels tab, select the Border Channel, Right click and select "Channel to Selection" from the drop down menu. This should give you a selection that covers the border area. You then need to go back to the "River Color Layer", not Channel, make sure you are selecting the "Mask" and fill the selection with Black. Then you should have a "River" Mask in your channels tab. Repeat the right click selection thing. Oh I see now, when you made your rivers layer, Black on white or whatever, all you need to do is select the rivers by color select or however you like and then go to the Select Menu and "Save to Channel" hope that helps.

@ BernieB: Thanks and good luck.