You are making an assumption that it was the US. You are also making an assumption is was one side or the other and not them working in tandem to tear down the system.

I highly doubt the EU would have separated itself from Russia if they didn't have or knew they couldn't have those reserves filled before winter. They could possibly damage any pipelines in the water if they want to retaliate if they thought it was the US or EU but those reserves are on land more than likely buried, they'd have to hit land to take them out. I am sure, just as with Russia there are cut off valves along the lines. But maybe that is what they are hoping for since they keep selling the Europeans as freezing to death this winter, as it stands now I highly doubt it but if they take out the pipelines it becomes more a reality.