When I was at the U of M completing my fisheries and wildlife degree we had a class where we tracked walleye and raccoons that had been fitted with a rf transmitter. We drove around lake Itasca in a pontoon with a big antenna and headphones listening to that good ol' walleye "beep-beep." Tracking the raccoons through the woods was done with a hand-held antenna.
I would think that's what is going on, gathering wildlife movement data, and the large antenna is for better range. Since they're on the river they might be tracking fish, or a mammal that stays in the valley like sasquatch.
I could be completely wrong about the wildlife tracking, but i know it's a technique used in the DNR/fish & wildlife for gathering data.