Haul every two years whether the boat needed it or not was my mantra. At least, if you bought a decent bottom paint. And in the Columbia even longer interval.

However, back in the early 1900's the Columbia did freeze over enough so's folks had pictures of tin lizzies driving across to the other side. With luck the link would take you to the images pages of Columbia and Willamette frozen over.

https://www.google.com/search?q=columbia+river+frozen+over&tbm=isch&tbo=u&source=univ&sa=X&ei=dXPuUvqZPMH1oASe-4DABw&sqi=2&ved=0CCQQsAQ&biw=1129&bih=643

In the area known as "the Gorge" winds come through at high speeds and in winter that east wind brings COLD. Crampons on your shoes to get your boat would be in order then. But no one to speak of pulled their boats. The river would still be fluid, as it were. Portland being somewhat at the tail end of the gorge area gets frozen streets from time to time. I've seen ice an inch thick on the roads and parking lots and driven in it many times back in the day I lived and worked there.

Icicles hanging from the hand rails kind of weather. Going to the boat to break the ice off the decks kind of thing. But no boat pulling. If we had to do it, it would be overwhelming due to fewer yard slots on the hard than boat by far.