I don't know of an official source, but if you can just drive by a mill. I've been by two and witness first hand the pile up. I've driven by one mill two times a year for like 17 years and they have at least 10x the milled lumber stacked up. I've never seen it even remotely that full. I say 10x but it could be even much more. The other mill had probably 3-4x what they normally had.

There was also a tariff and import cap put on during the Harper era.