At 5000 there really isn't anything in worth
going to Trestle, however around 6000 to 7000 it
can be a fun little wave.
Between 7000 and 8000 it starts to break on
itself . 
Above 8000 it gets really sticky... all the way
to 20,000 cfs. Spring is the time! 
Yes, you still have to buy a permit (it's weird
to be surfing and get called over to the shore by 
the Park ranger so he can check to see if you
have a permit).  
Hope this helps.