If you come from Garnet Lake and reach the San Joaquin River in late spring or early summer during snow melt, you might not be able to cross it safely to get to the Thousand Island Trail on the east side. There is no bridge and the tree trunks are not to be trusted. As an alternative, you can hike a spur trail that you will not find in a paper map, but trust me, it is there. Follow it and you will get to a section where you enter the trail to Shadow Lake. The spur trail is not maintained and sometimes overgrown, so you might need some route finding skills. If you want to cross the river before the spur trail intersects with the Shadow Lake trail, there are some spots, where the river is less raging.
There are also some nice spots to pitch your tent, if you plan to stay here.