How does the game decide which map will be used for each battle?

How/when does the game choose to use winter maps?

Why, if a border between two provinces has a River, which is displayed on the info parchment (sometimes because I tweeked it), do battled between those two zones not actually use a river map?

My example:

I decided that battles faught between Wessex and Flanders should be fought on a river map. I went into the Early.txt file and in the border entries between Wessex and Flanders I changed both to RIVER. This is displayed when you right-click on one of those provinces and move the cursor over the other, it says there is a River Waterway. However, England just attacked me (France) in flanders, and the battle was faught on a map without a river.

Also, I found, further down in the Early.txt file, another set of data on what kind of map and terrain battles within each province will be faught on, though this time the game didn't seem to care from which direction the attacker came.

Do certain territories use river maps no matter from which other territory they are attacked from, or does the game actually take into account whether or not a river actually exists between the attacking and defending provinces?

If the game does differentiate, where do I need to make the changes such that, say, battles between Wessex and Flanders are fought on river maps but battles between Flanders and Lorraine are not?