I assume they sent the water over the Canadian side simply due to the treaty. That said, they have excess capacity to handle blocking 1/2 the flow over Niagara Falls every night, but even doing nothing was still a non issue.

If you assume they blocked 1/2 the flow (1,400 m3/s) and didn’t use it for anything. That would still take 21 days to raise water level of Lake Erie by 1cm.