Ive paddled the NF Blackfoot twice, once at around 1200 CFS and once at like 450. 1200 is legit Class IV, my guess from that flow is that somewhere around 1500 or 1800 it gets a bit technically easier but the consequences of a swim continue to get worse. I can’t access the USGS data for the flow on the first day of the open in 2018, but from my memory of the hike up I think that whatever that flow was is probably the province of big water Class V kayakers.
I probably wouldn’t get on it alone above about 1800 or with a partner above 2500. For many other packrafters those cutoffs are likely to bet lower; I think it mostly depends on your comfort level in the boat. Of course we wont know the flows to that level of detail having spent the last 36 hours in the backcountry, but it’s a useful reference I think. If you’re dropping in from the North (and so can’t scout the river on the hike in) the availability of a wood report from the current season is also a factor in the decision matrix.
Below where the NF Blackfoot comes out of the mountains it looks like Class II, ditto the Blackfoot proper. So probably fine during runoff as long as you’re heads up about wood. But other folks who have paddled that stretch will probably have more info.
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