Packraft forecasting

It is that time of year; orders for straps and the guidebook start increasing, as do emails about trip planning.  Those messages generally involve a ~7 day hiking and packrafting loop in the Bob, and almost always revolve around attempting to plan well in advance and hit a reliable flow window.  This is especially tricky for something like the South to North Loop, which for less experienced packrafters requires balancing enough water on the North Fork of the Sun with not too much on the South Fork of the Flathead.  

I provide guidance on flows in the guidebook, as well as guidance on when those flows are most likely to happen, but for a specific year doing specific research is invaluable.  The main Snotel page is the place to start.   I like to use the percentile compared to POR option in the interactive map, and especially the water year chart for individual Snotel sites.  These charts reveal both how water is accumulating relative to the historical average, and when over the span of variability total snow water equivalent (SWE) peaks and, thus, meltoff begins.  In short, big snowpacks can make for big and or late flows and or sustained streamflows, but how the spring months (starting now) play out is almost more relevant.  Temps cold enough to have April and even May storms fall (above 5-6k) as snow can make a below average winter into an exceptional spring and summer.  

The next step is the streamflow pages, specifically the monthly averages.  This gives you a good sense of the potential for variability, something that you can then go back and correlate with past snotel graphs, which is the best way to cultivate a depth of context.  The North Fork of the Sun, for instance, has a reliable period when it is floatable, and a reliable period when it is not, without a massive amount of variation.  What variation there is directly correlates with overall low snowpacks  (eg 2016).  The South Fork of the Flathead, by contrast, has a similar degree of predictability when the low limits of floating are concerned, but much more variation when higher flows are concerned.  My standard threshold for when the lower White River ceases to be good floating is 5000 cfs on that South Fork gauge, and when that boundary gets crossed (usually in July) is highly subject to conditions.  The trick here is that there is not a snotel site within the main South Fork headwaters.  You’ll need to look at places like Badger Pass and Noisy Basin and extrapolate, again based on specific comparable instances from the past.  

Flow planning such as this can make taking a trip in early July versus late July an easier choice to make, when the choice is being made right around now.  Closer to the date of a trip, say a month out, possible flows can be more reliably forecast, making it possible to (for example) route from something like the White over to the Danaher if flows will be lower, or to plan on portaging the lower Youngs Gorge if flows are higher than a group might like.  These two tools can also be used in other locations, though local idiosyncrasies* will always throw in curveballs.

*The only gauge on the Middle Fork of the Flathead is 50+ river miles downstream from relevant wilderness sections, with the highest altitude headwaters (Park, Coal, Nyack Creeks) coming in below the wilderness bits.  In July and August the wilderness Middle Fork is consistently lower than the gauge would suggest.


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