I think over 10k on the Queets is the general rule for Metheny. Be sure to check out the Metheny page- it is probably reliable at least in a general sense.

If you use the side creek (I forget the creeks name- its probably on the metheny page) put in way up, it'll be a skinny sketchy IV with gnarly mank ugly V fu*k up potential and often at least a bit of wood or more. As far as the stuff on Metheny Creek itself, I don't remember it having a strong class V flavor. There may be a little class V spice sprinkled into a few spots up near the top of the upper, but it is largely a class IV dish that tapers down to class III+ desert. Obviously added flow will make everything half a class harder or more depending on how much H2O you add.

As far as wood Im not convinced the OP has any more per se. But in my experience there is certainly more wood on the roads. Winds knock a lot of sh*t down out there (so yes, I spose there prolly is more wood in those creeks, esp. Those in the SW quadrant of the OP). Anyway bringing at least a bow saw (all I ever had out there) or even better yet a chainsaw is a good insurence policy that you can get in (and out of) your access points. High clearance rigs that can rally over low wood can't hurt. Bring plenty of tarps, dry firewood, and fuel to light it. That in essence, are the extra requirements for trips out there if you want comfort and ease. My fantasy set up would be a medium sized RV with a Jeep and have another sturdy 4wd rig out there too. 3-6 people and everyone can sleep and cook in the RV... But, now I'm just dreaming.

You can camp along the humptulips, and the put in for lower metheny (take out for the Upper/Middle combo) is right by the bridge. Prolly better for car camping tho. Take out for lower metheny may be better.

Hmm.
in this case PP's database for Matheny is pretty scant. I'm surprised. Its a popular run.
Megan, if you'd like I can dig up my river log and PM some beta to you. Let me know when u are going out there.

Humptulips is rad. Anyone who is feeling solid on the class III should definitely check it out if they got someone who knows the run. On of Washington's class III GEMS for sure. Great "adventure class III" due to its remote nature and canyon setting.

As of last weekend, Matheny was amazingly clean.  The only portage on all three stretches (upper / middle / lower) was a step-over of a log in the upper section just above the drop leading into Sharks Fin Falls.  Also worth noting is that the Humptulips (all three sections) were clean earlier this fall.  There's a bit of wood collecting at the end of the gorge on the "Gorge" run, but there were a couple of options to sneak through this in your boat.  And the Wynoochee gorge was clean as well.  If it stays sunny and cold, not sure Matheny will have water.   

How low is too low in Matheny?

All three sections can be ELF'd, and it's good ELFing - if you like ELFing...  As Jed mentioned, the Queets is the USGS gauge everyone uses, and while it's not an exact correlation, it's better than nothing.  Assuming you're not dealing with low snow levels, 5K on the Queets correlates to nice lowish level (not ELF low) on Matheny.  Sometimes 4K on the Queets feels the same, sometimes it doesn't.  I've done all three sections in the 3700-4K, and that's getting ELF.  Still enjoyable, but ELF.  I know others who have been in there with the Queets at 2500cfs and said it was still worthwhile.  That may be because they just drove 3.5hrs to get there...  Above 5K, the lead-in to the drop zone on the upper cleans up, as do some of the in-between sections of the middle.  Somewhere in the 7K range on the Queets is normally pretty sweet.  10K can be sweet, and can be be getting pretty high - it depends.  Of course, there are some nice playspots on the lower run at high flows.  Using the rocks below the West Boundary Rd / FS Road 21 bridge (upstream side of the bridge), if the smaller (RR) of the two rocks has water flowing over it forming a wave, you'll have good levels.  If it's forming a hole, or if it's sticking out with an eddy behind it, things are low.  If the bigger of the two rocks has water flowing over it, hang on.    FYI - when I do the upper, I no longer bother with the pinball scrape fest down the side creek from the closed bridge.  Instead, I walk across the bridge, up the road to the corner, cut into the woods, and traverse to Matheny proper.  The trick here is to not get "sucked" towards the side creek to your right.  If you do this, you'll end up dealing with "the swamp".  Stay just left of the swamp (you can wander through the left edge of it if you don't mind a bit of muck) and you'll be on a good path.          

Hey Megan- I just sent you a PM with some info from my river log, although Im not sure how useful it is. Reading my Metheny entries it looks like I was slackin on those. But Almquist has probably run it as much as anyone around, and the Whitewater Intellect of Mr. Hawkes is sharp.

Looking at the 10 day outlook tho it doesn't bode well for water. I would think you'd want to see at least steady moderate rain to justify a trip out there. Queets is at 4,000 which is getting into that ELF flow range. But the Humptulips looks like its holding decent water...

Since PP's database and my own river log entries are pretty vague, here's a quick quote from the master himself, Mr. Gary Korb...

He calls that little side creek put in
"The North Fork" (I can't find my topo map for Matheny, either)

He writes:
"From here you have two options, the first is just running the creek. You'll hit about 200 rocks and pin several times, but its fun! If the water is low the other option is to cross the bridge..."

- from "A Paddler's Guide To The Olympic Peninsula" by Gary Korb

Go for it. Its past my bed time and I got bigger fish to fry...