For the Six-Mile Gap landing, the feature to look for is a rock tower on the left bank. The tail of Stovepipe Rapid has cliffs on river right and a rockslide on the left. You can see the channel slope decrease downstream as the river divides around a willow-covered island.
The beta above is right on. Keep hard left after the rock tower. Pulling into the rocky left channel you can see the trail on the left.
Here's a copy of an e-mail I sent in response to questions about the North Platte, with a few added blahs. Also, a link to the FS website.
USDA Forest Service, Medicine Bow & Routt National Forests, Thunder Basin National Grassland - Recreation Activities
Is there good camping in the Northgate section?
There are camps at the mouths of tributaries (mostly right), mostly with steep banks, willows: nothing I'd call good. Very few people camp since it is most often done in a day to Six Mile Gap. At higher water the camp spots disappear.
Would I be able to get my 15 footer down and at what minimal level?
I've run a narrow 15 ft. cat (with two people and no load) down at 850 cfs and it was enjoyable but a lot of work, dodging rocks. For fun, 1000 cfs is a good minimum. The FS recommends 1200 for loaded rafts.
What is the mileage to Treasure Island?
Don't know. I figure Routt put-in to Six Mile Gap is about 4-5 hrs. with a lunch stop, at medium water (1000-1500 cfs). Best bet is to camp the first night opposite Six Mile Gap or below (The Waterfall camp is nice, but small). There's a lot of private land lower down, but there is a FS camprground on the right bank just above the private land boundary, and more public land with good riverside camping on river right, opposite the Big Creek confluence. For boundaries and ownership, get the BLM 1:100,00 series map for Saratoga (yellow cover).
Could I do both the Northgate and Treasure Island as a one nighter?
Yeah, but it'd be a push. At higher water (1800+ cfs) you flush through pretty fast. Better to camp below Six Mile the first night and between Corral Creek and the Bennett Peak ramp the second night. That gives you a half-day run to Treasure Island for the take-out day.
For a one-camp trip to Treasure Island, I start at Six Mile Gap (have to carry down a trail). For a one-nighter from the Routt ramp, the best take-out would be the Bennett Peak ramp. But the shuttle's a slow grind over dirt roadsó don't do it the first time in the dark.