Yeah, you would need to stack em in that case.
Is it possible to cantilever your dock out from the shore?
Have good piles on shore say 10' apart. One set right by the shore and the second set 10' further back from the shore.
Cantilever 4' over the river with no piles in the river.
If you suspend it over the river you shouldn't have issues with ice or high water unless it's stupid high.