Seives are present at low water
The seives are especially bad at flows below 600 cfs. The higher the water, the more filled in, so I believe the danger is somewhat lessened as long as you stay in your boat. Flows over 1100 (my highest run down the UTB)would start to feel very big, and most of the rapids would be totally filled in. That being said, I wouldn't want to be out of my boat very long anywhere in that section, at any level. Sieves are still a problem at NCO (very bad) and Hell Hole; and Big and Little Arsenic are always sketchy. Current flows will be very challenging (think Gore at 1400). Go down with someone who knows the run. Wear a face mask as there seems to be a lot of sharp rocks around.