West of Mammoth Lakes, the middle fork of the San Joaquin River is a very popular area for fly fishing. In other words, if you plan to go, make sure you get there early to stake out a spot. If you arrive after the cutoff time, you will have to take the shuttle bus down into the valley. It is a designated wild trout river and is known for its brown trout, golden trout, rainbow trout and brookies.
The river is accessed from Mammoth Lakes via Minaret Road (aka SR203). The most popular section is the two mile stretch around the three National Forest campgrounds.
Overall, San Joaquin River is the 3rd most popular fly fishing river of all 17 fly fishing spots in California.