D&D 3.5 Reflex Save or Balance Check?

[ShadowfireOmega]

I was looking around in the PHB and I cannot seem to find which of the two to use for sudden stops on moving objects. My players are going to be on a barge that is part of a 'barge caravan', when all of a sudden the first 5 (of 10) barges are going to be suddenly halted due to goblin raiders and trap including very large chains tied to very large trees. I am going to have it to where they must make a save or check in combat round 0 to avoid being knocked prone, and must make this save again at rounds 5, 10, 15, 20, and 25 as the other barges knock into them, would this be a reflex save or balance check?

[Curmudgeon]

Saving Throws

Generally, when you are subject to an unusual or magical attack, you get a saving throw to avoid or reduce the effect. The barge stopping isn't an attack.
Action: None. A Balance check doesn’t require an action; it is made as part of another action or as a reaction to a situation. Balance checks can be made when it's not the character's turn, so they appear to be suited to this scenario.