I am unsure whether I have interpreted the rule correctly concerning the effect river hexes play in movement.
The rule says, " A unit which enters a River Hex must expend one additional movement point to leave that hex through any hexside which it could not reach without physically crossing the line symbol which represents the river."
So a test for this if you draw a line from start hex to finish hex through any part of the hexes the unit entered. If that line must cross a River line, you probably have to pay. Put another way, could an ant walk a path though those hexes from start to finish without crossing the blue line?
Hence, if an infantry force occupies Y10(4) at the beginning of the movement phase then its first movement point will get it to Z9(4).The force then moves to AA9(4) for one movement point because it has not crossed a hexside representing the river line symbol i.e. the hexside shared by Z9(4) and AA10(4) is clear of a river line symbol.
No, Y10 to Z9 is 1 MP, Z9 to AA9 is 1MP + 1 extra MP because to move from Y10 to AA9 requires crossing the river.
The infantry force ends its normal movement in hex BB8(4). If this is correct then River Hexes become effecting terrain only if a force moves upstream or downstream during a movement phase.
This is incorrect, the hexside depiction has nothing to do with this.. The determination is between start hex and finish hex. It is in fact possible to move along a River line and never pay the extra 1MP if both the start & finish hexes are conceptually on the same side of the river. For example, Y10 to Z10, to AA11 to BB10 doesn't cross the river since you can trace a line along the southern side of the river.
Hence, if the force at Y10(4) moves into Z9(4) for 1 MP then movement into Y9(4) would cost 2MP since the movement physically crosses line symbol of the hexside shared by Y9(4) and Z9(4). Normal movement would end at Y9(4).
No, the move from Z9 to Y9 need not pay 2MP.
The second part of the rule states that, "A unit which occupies a River Hex at the start of its movement phase may enter any adjacent hex at the normal terrain cost. So a force starting movement in Z9(4) could move into Y9(4) for 1MP .
Yes, and could then move off the river at the cost of 1MP in either direction. Once it leaves the river line, it cannot escape the cost of crossing the river again. So for example Y9 to Z9 (1MP) to AA10 (1MP) to BB10 (1MP) is fine, but a force march to CC11 would be 2MP.