It can be. Really it's more about the weather than it is about the location. There hasn't been much slush the last 2 weeks, but there is fresh snow on the top and it it gets warm, then snows again... Then it and everything else is going to get slushy.
I fished Granby this weekend and there was plenty of slush and it snowed the entire time. My guess would be Williams fork got at least as much snow if not more. Iím sure there are some slushy areas and some with just deep snow.