You can tell if an invasion will have a bridge on the campaign map by popping open a province details scroll, and moving the mouse around the bordering provinces. Text in the scroll will appear about attack details.
The MTW wiki has the map types for every border attack/defense/assault in the game, just go to the province. Khazar is a good example, it's got plenty of rivers.
You can also look at the EARLY.txt file, river maps are chosen based on the SetBorderInfo:: lines. Quick grep gets the following list:
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