During winter game turn, Army Forces keep move within frozen major river.
De facto, it seems like frozen major river are considered equivalent to land region for the aim of Forces movement.
At first I noticed that with AI forces, then I tried to move player forces there too.
The example: Sun of Austerlitz scenery
I have been able to move a Corps with ISAR CONFLUENT.
In next turn I ordered it to move from ISAR CONFLUENT to INN CONFLUENT.
It seems like this aspect has not been reported in game manual: Version 6.2 for NCP 1.03.
Also it usefull to clarify movement, attrition and combact effect of frozen rivers over Armies moving & engaging over them.
JastaV