Keep in mind that this is just a guide and individual skill and fitness levels should be your real guide to safely wading and/or floating any river. River flows are described in cubic feet per second (cfs). 

The West Branch

400-450 cfs:  Almost 100% wadable.  At this level drifting will mean walking the boat over some shallow areas.

600-800 cfs:  At best, 60-70% of the river is wadable.  The river becomes very floatable.

1000-1200 cfs:  Except for the most aggressive waders, not much of the river is wadable.  Fishing from a boat now becomes the best method.

1200-1500 cfs:  This is the highest level you should even think about wading.  Even so, the places where you could safely wade are few.  Drift boat fishing is ideal.

1500+ cfs:  Yikes!  Don't even consider wading.  Too high and too dangerous to wade.  Very fishable from a drift boat.

Upper East Branch

The Upper East Branch is that part of the river north of NY Route 17 (upstream from the Beaverkill).

250-500 cfs: At this level 75%-95% of the river is wadable.

600-800 cfs:   On average, about 50% of the river is wadable, maybe 60-65% at the lower end of the flow range and less than 50% at the higher range.

900+ cfs:  Don't wade. Too dangerous.

Lower East Branch

The Lower East Branch is South of NY Route 17 (downstream from the Beaverkill).

700-800 cfs: 90% wadable.

850-1000 cfs: Still wadable in most sections, but at this level the river also fishes well from a drift boat.

1200-1500 cfs: Less than 50% wadable.  The river is more fishable from a boat.

1600+ cfs:  Drift boat fishing.  Too dangerous to wade.