In winter, the boardwalks are pulled out the gorge, and as soon as the stream freezes enough to provide places from which to belay, ice climbers move in to play on the ice that coats the southern wall of the canyon.
Though not tall enough to rate a mention in, say, Lewis & Horowitz' Selected climbs in the Northeast, the Flume Gorge is a well-known spot and an ideal practice ground, with its convenient location near the highway, ample parking, and easy toprope setup.
Routes here probably have names and consensus ratings, but I don't know them. The obvious flows look and feel like WI4 to me, with some stiffer variations (including a bit of mixed) possible. See the photos.
Toproping is the norm here, and the only way to meaningfully protect some of the thinner and more fragile lines, but the fatter flows can also be led.
The most convenient access to the top of the south wall is via a diagonal ledge whose low point is at the entrance to the gorge, specifically at the last bridge before the boardwalk is removed.
Geologyhuge chockstone once bridged the gorge, but it washed away in a flood in 1883. The sides of the gorge are mostly dead vertical, and are formed of solid granite with few of the typical signs of stream erosion - no wavy forms or pot-holes here, just a little bit of rounding off of the corners of some cracks. The stream has not worked any great excavations above or below the gorge.
The geological explanation is that the Gorge is a sort of fossilized remnant or cast of a basalt dike. Magma once flowed into a crack in the granite, widening it slightly. When this formation was exposed to the surface, the basalt (ex magma) eroded far faster than the granite, leaving the empty crack behind. Today a bit of basalt is still visible in a cave at the head of the canyon, east of the waterfall.
Getting ThereFrom interstate 93 northbound, take exit 34A (inside Franconia Notch) onto route 3 northbound. In less than half a mile you'll see the huge sign and parking lot for the Flume Gorge Visitor Center. Park here. Watch out for snowmobiles in the parking lot.
Make your way around the visitors' center (there should be at least one herdpath, just watch out for possible thin ice on the trout basins by the entrance). From behind the visitors' center a road and trail make their way northerly and downhill to a red covered bridge. Cross the bridge and continue following the road and trail uphill to a white building, which in summer is the terminus of a shuttle bus. Follow the very obvious trail past the white building and alongside the stream. (Ignore some side trails for now. One of them is the return leg of the trail, which runs along the top of the north side of the Gorge. Another leads away to the Pool, which is worth a visit but not the subject of this page.) Stay on the trail until the boardwalk abruptly ends at the west end of the Gorge. Revel in the glow of flashbulbs (no kidding - even in winter there will be car-tourists) as you ignore the "stop" sign and lower yourself a couple of feet onto the frozen stream. Walk through the gorge until you find a climb that's not already occupied.
Red TapeThe gorge is inside Franconia Notch State Park. Parking is free and (most of) the lot is kept plowed.
No camping inside the state park (not that you'd need to), except at Lafayette Campground.
The Visitors' Center is open May through October and charges an admission fee ($13 for adults as of 2009) for access to the Gorge.
Weather & When to ClimbThe Gorge is so narrow that it's very sheltered from the weather, and there's no avalanche danger.
The limiting factor in early season is the stream at the bottom of the gorge, which may not freeze sufficiently for a decent belay until January.