Mutau Flat trail is the Johnson Ridge route. I hike that area quite a bit and I do not believe Piedra Blanca will be that bad. The Sespe there will have two crossings in the riverbed. My guess is that if you take off your boots you should be able to wade across. I have never seen it deeper than a couple of feet and maybe 20 or so feet across. Other crossing along the trail you can do the same or they even have large rocks to help you manuever. Except right after a downpour and then I would not attempt it.