roots on trees you want to keep. And for that matter, you don't cover them with dirt either. I think you can smother the tree if you add a bunch of dirt - I don't know how that changes if the creek just recently washed out a bunch of dirt. But that tree looks to be at least 20 years old and I'm guessing it's roots are familiar with the changes in creek behavior.

So...I know nothing except my experience and that is if you mess with the roots the tree may not like it. Small ones I've never had trouble with, but those all look like big healthy roots.