Thanks Rich - that's pretty sobering because I'm not sure where the state line gage is, but the photo on the USGS webpage makes me think it's fairly open with lots of sun. If there's ice at a place like that, then I'd certainly expect ice dams where the canyon's are narrow and shaded a lot.
I had some buddies awhile back that wound up caching their boats and portaging with only a little mini-me sized boat (the Rhino) around a huge ice dam on RHT once. All of them floated out the tiny boat next day. They had to come back after the river thawed and get the rest of their boats and gear, I think it was around Valentines day that year. There were some great photos of them dragging the frames and gear through the snow that brought to mind Scott and Amundson.
I've heard that Debeque Canyon is a good proxy for Westwater and RHT - if you see ice while driving through the canyon on the way out to GJ, then there's likely ice down in Westy and RHT. That said, Debeque isn't nearly as deep and narrow as the inner gorge of Westwater.
Even though I'm not a real fair-weather boater, the thought of needing an ice axe and crampons on a rafting trip just seems like a bad thing no matter how you slice and dice it....
Rather than a bunch of internet arm waving and conjecture, you could always call the BLM in Moab and ask them.
The skiing really isn't so bad right now.
Good luck if you decide to go for it.