USGS Gage Station CFS near Alcova.
This section of the Sweetwater is terrific. The beginning and ending stretch is a Class I, but the middle area through the canyon is Class III. Part of the canyon area has recently been purchased by the Nature Conservancy and access routes may change. (The canyon was once proposed for Wild and Scenic River classification.)
The put-in is reached by turning off W-28 at mile #45 and heading toward South Pass City. After two miles you come to a junction where you take the Pick Axe Road south toward Willies Handcart Site. Stay on the main road south for 11 miles to the Three Forks/Atlantic City Bridge. Just past the bridge is a BLM section of land and a two-track road turns east to a loop in the river at the put-in.
The river meanders considerably for the first two miles until you pass Long Slough coming in from the right. The river then straightens bit and the valley narrows. Two hours from the put-in is Wilson Bar and you enter the canyon. This Class III area has some stretches of rock gardens and in places there are large boulders to contend with.
After eight miles the canyon ends and the river starts meandering again. Just downstream, Chimney Creek comes in from the left (the old chimney is still standing) and down a bit further is the Arnold Diversion Dam. Two hours from the dam is a small bridge and a few miles further is where Alkali Creek empties into the Sweetwater. You then have another small diversion dam and go under another small bridge before the take-out at the Sweetwater Station on US-287. (There is no public access point here, so you need to ask permission to trespass from the good people at the River Campground.)