I think a sluice box scenario is applicable. The force of the moving water has to be less than the baffles ability to hold the heavier material in place. If there were no baffles, all the material would move downstream, some maybe a bit quicker than others. I think any terrain that is durable enough to remain substantially unchanged by wind forces is not acting like typical pieces of lead shot.

I think a reasonable question for an anti lead hunter/shooter is, what are your motivations, what are your goals. If they're reasonable and attainable, how could there be opposition. One might be forth coming and factual about intended and unintended consequences, not just looking for the satisfaction of a 'win'.