Staircase

Without a doubt Staircase is one of Sixmile's signature rapids and at some levels its hardest. The river bends right and breaks around a large boulder island. Most of the water goes right and dumps over a steep ledge that can form a meaty hydraulic at medium flows and up. Not that you want to swim here because just downstream the river vaults over a steep chute that ends with a stout ledge. Above 10.0' kayakers can boof left of the island to avoid the right side hole. Either way you still have to contend with the rapid's bottom half. At low flows, the bottom ledge forms a powerful and sticky hydraulic that is hard to miss. This hole washes out into a turbulent maelstrom of haystacks and laterals above 9.8' or so. 
Although often listed as class V in many sources, at most commonly run levels Staircase doesn't feel like class V by modern standards. Perhaps the "Alaska factor" (cold water, remoteness, lack of EMS services) may contribute to its higher grade even on a roadside run like this. The rapid does seem a bit worse at low water when the hole is stickier than at medium when everything starts flushing through. All of Third Canyon should be treated as class V when it gets around 10.8' (2,500 cfs) and higher. 
Staircase can be scouted and portaged on river left and can even be pre-scouted during the shuttle via a steep trail (there is usually a fixed rope to use as a handrail for the steepest section) found at mm 5.3 off the Hope Road.