I would check in Spokane or Lewistown, maybe moscow[U of I, or call nrs and ask] depending on route. I think there is a scat machine in Lewistown, or nearby to serve the lower salmon. You probably won't want to backtrack to Missoula after your trip esp with no scat machine along that route. I have dumped groovers at the Missoula wastewater plant, but I th ink that was under the radar.
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Living in Montana, boating in Idaho