FATMAP difficulty grade
Day 1: Late start after lunch in Bellingham, hike from Hannegan Pass TH to Copper Creek campground. Day 2: Hike from Copper Creek, wade across Chiliwack River to old trail up Easy Ridge.
Across Easy Ridge, down into basin below Whatcom Peak's south face, navigate the Imperfect Impasse, and up to camp at Perfect Pass. Day 3: Hike from Perfect Pass across Challenger Glacier and up to summit rock.
Retrace steps back to camp, climb south face of Whatcom, downclimb north ridge and eventually find hiking trail, hike to Graybeal camp. Day 4: Hike from Graybeal camp to Hannegan Pass TH. This track includes info about finding the infamous Imperfect Impasse.
In words: Follow Easy Ridge, until you are close to Whatcom Peak, but drop down onto snow whenever convenient- there's no advantage to fighting to keep your elevation until the bitter end.
You will drop down quite a ways to go below a buttress coming off Whatcom.
Contour around close to this buttress and you'll eventually wrap around until you see the buttress toe into some flatter ground with mature trees.
Right here is a view into the gaping chasm, with sheer rock walls on either side and a waterfall visible just above you.
If you stand here you've come too far but not by much.
The way across is about 300 feet above, where you can climb into the gully above the waterfall.
Make an improbable-seeming traverse up and right across 3rd class slabs, until it's possible to wrap around a corner on good holds, and into the gully.
Climb this until it's convenient to exit onto slabs on climber's right. This track also shows you how NOT to get off the North Ridge of Whatcom, I strongly recommend against getting greedy like we did.
We should have stayed on the ridge until we hit the trail at Whatcom Pass, it would have been much faster and easier.
We were very lucky we didn't have to climb back up and try again.