Great fixes on the .ter files to address the frozen river = highway issue. However I seem to have just had a rather large land battle (Grant vs. Johnston) on the Mississippi, specifically Cape Girardeau river location. about 10,000 USA vs about 6,000 CSA. I'd be happy to send along my log files. This is a non freezing area, Johnston was at Cairo and Grant was coming in on river transport (the "button" kind, not on specific ships). Perhaps they exchanged fire from their river transport....although I'm not sure that should really happen.
EDIT: I've also got artillery moving down muddy rivers (no river transport being used) with the efficiency of a railroad. In fact it uses the river only when I select use rail for movement at the rate of rail movement. All these odd things happen during the winter thus far...did not notice anything during warm months.