From Thousand Island Lake, you can retrace your steps a mile back to the junction of the Thousand Island Lake, River, and High Trails. This time take the River Trail, which immediately starts heading downhill.
This trail is 7 miles between the junction and the trailhead at Agnew Meadows. As the trail begins, you get a few views of the Middle Fork San Joaquin River, which the trail follows; there are later views of giant granite ridges rising above, and San Joaquin Mountain and the Two Teats. The trail also passes a couple of small waterfalls on the river, then passes the trail junction to Garnet Lake. Contuining, the trail is quite boring; there are only a few open spots with rare views to the Minarets. About 2 miles from the trailhead, the trail passes the junction to Shadow Lake; from there you can see Shadow Creek cascading down the granite to the Middle Fork San Joaquin.