Oarboatman is on point. Mid-May might mean a launch on Marsh - @ 90 cfs. Thankfully another small creek joins in after 100 yards and now you have 150-200cfs! Thankfully it is a small channel and people get 16' boats down Marsh with some effort.
By the lower sections of the Middle fork you could be on 5,000-10,000 cfs on the Middle and add another 10,000 cfs at the confluence with the Main. Snowpack this year is on the low side, but it might still be a couple weeks before you know where you are likely to launch.
I've taken my 7' wide 14' long self bailer (heavily loaded) down Marsh several times with 9' oars. I usually get hung up a couple times. If you haven't been before it is technical boating with blind corners and the possibility of channel blocking logs in some places. Frankly the whitewater doesn't exceed III+/IV- on Marsh (and really in only one rapid), the real problem is potential for huge logs with sharp broken branches blocking the channel combined with a lack of eddies. People have died on Marsh due to the logs. That said, the three trips I've been on we didn't have wood problems on Marsh. One trip Dagger Falls was blocked by three massive logs. Another trip after the Lake Creek blowout we portaged all the boats around that due to wood issues. Pistol rapid also likes to hold wood; a massive log jam there eventually had to be dynamited out.
If you haven't joined the Idaho whitewater yahoo group ( https://groups.yahoo.com/neo/groups/idahowhitewater
) do it now. There will be updates there about levels, snowpack, road openings, maybe flyovers of the river before the first people go down it, and trip reports. Make sure you file a trip report there as soon as you get off the river if you find any issues with wood - or to report a lack of wood issues.