Here in CO we have no clear cut law either way. Is the fence illegal? Maybe. Is the landowner opening themselves up to a TON of liability issues? Most definitely.
Was there any mention of fences on MB, Riverbrain, American Whitewater, Eddyflower, etc.? NO, it was quite the shock to us as it seemed like this was a run that was done regularly.
There is evidence of the creek being used as a waterway (including kayak races in the 80's) and as long as you don't touch the bottom or the banks, you (as I interpret the law) are good to go. FYI- the landowner does not run cattle on his 4 acre parcel that he bought in 2006. There are NO cows. Even if there were, the landowner could make it passable through use of PVC pipe on the lower wires, smooth wire instead of barbed, vertical slats, etc. Also, on a practical note, as a landowner myself I wouldn't want my fence to catch all of the debris coming downstream and get torn to shreds. Rebuilding every year seems costly and ineffective.
Here is a link to Mesa County GIS showing the property in question (parcel 12513 and his 50m +/- of waterfront):
Here are some references that I found after I got home and was fuming mad:
Rancher Forced to Remove Razor-Sharp River-Fence of Death
Can a Landowner Fence Across a River ? - PLWA - Public Land / Water Access
Landowners' river fences pose ongoing rafting issue - The Denver Post
It is obviously an unsettled issue that will continue until the courts make a defining ruling one way or another. Keep your heads on a swivel fellow paddlers and your snips handy!