Moros helped me today a lot by teaching me how to create a river in the middle of the map, but when all territories are drawn. So, he send me a tutorial in my mail and now I want to share this with you:
Draw a circle of 16 by 16 pixels. Make sure it has a border that is 1 pixel thick.
This circle is about the size of an army number in a territory. That makes it easy to check if a territory is large enough to fit a two-digit army number: if the circle fits, the number will too.
Draw the line where you want to make a river.
Set the line width to 3, 4 or 5 pixels, whatever you prefer. Then select it and press CTRL+ALT+C.
Select the river, duplicate it (CTRL+D), hold shift, select the river and any neighbouring territory, press CTRL and -.
This will put the border at the exact right spot. Do it for all territories.
Use the circle to check if the territories that became smaller because of the river still fit army numbers. If not, move some nodes to make them bigger to fit army number before uploading the new svg.
Upload the new file to the WL editor, and make sure to move the centerpoints where neccesary.
How it looks on my map: http://warlight.net/SinglePlayer.aspx?PreviewMap=25260
~All credits go to Moros~
Edited 9/22/2013 11:25:37