My recollection of the main rapids from the Northgate ramp to Six Mile Gap (with a few notes) are:
-Windy Hole: around first left bend, left run, nice bounce.
-Cowpie: over an hour down. Rocks get tighter over a quarter-mile, with the crux a right around the cowpie (big mushroom-shaped boulder) then a hard left (oars can hit rocks) followed by tail waves that break at higher flows. Continues to-
-Narrow Falls: cliffs pull in, ledges and large boulders funnel the flow to a center slot, with a steep drop and an immediate pull right. Left is a cliff face that flips boats, with a rocky swim below.
-Tootsie Roll: large boulders overlap somewhat, with pourovers and a sneaky hole low on the right (hard to see from the top).
-Stovepipe: rock garden that forms holes at high flow. At the tail, stay left to make the landing at Six Mile Gap (the channel splits).
There's a board at the Northgate put-in with a rough map- good for relative distances. At 1000-1500 cfs, I'd rate it 3+, adding a bit as the flow increases. 'Til you learn it, it's probably wise to have at least 2-3 boats along. Above 3000 cfs, Narrow Falls flips quite a few rafts: call it a 4+ or a 5. At 5200 cfs (10 yr. flood, in 2005) most boats flipped and some people took long swims. The water's cold and the canyon shady, with few good landings above Six Mile Gap: I'd recommend a wetsuit–minimum– or drysuit.
Welcome to Wyoming & have fun, mate.