The Crown Jewel of the Ansel Adams Wilderness
Thousand Island Lake spreads out beneath the twin summits of Banner Peak and Mount Ritter, its surface dotted with dozens of rocky islets that give the lake its name. Sitting at roughly 9,800 feet in the Ansel Adams Wilderness, it is one of the most photographed alpine lakes in the entire Sierra Nevada. The reflection of Banner Peak across the water at sunrise is the shot that draws hikers from all over the world. Both the John Muir Trail and the Pacific Crest Trail pass along its eastern shore, so the lake sits at a crossroads of legendary routes.
Where the Trail Starts: Agnew Meadows
The most common day-hike and backpacking access is from the Agnew Meadows trailhead near Mammoth Lakes, at about 8,300 feet. In summer you reach it via the mandatory Reds Meadow shuttle out of the Mammoth Mountain Adventure Center, since the narrow road is closed to most private vehicles during peak hours. From Agnew Meadows you have two main ways in, and they make a perfect loop.
- River Trail - follows the Middle Fork San Joaquin River, gentler grade, more shade
- High Trail (Pacific Crest Trail) - climbs onto a sunny bench with constant views of the Ritter Range
- Loop option - go up one and down the other for roughly 14 to 16 miles round trip
How Long and How Hard
As a day hike, Thousand Island Lake is a big day: plan on 14 to 16 miles round trip with around 2,000 feet of climbing, depending on your route. Most people make it an overnight or longer backpacking trip instead. The grade is moderate rather than brutal, but the altitude makes the effort feel real, so build in time and water. The High Trail is the more scenic ascent, traversing a hillside with uninterrupted views of Banner and Ritter, while the River Trail is the kinder, cooler descent.
Camping and the JMT Connection
Camping is allowed but tightly regulated to protect the fragile shoreline. You must camp at least a quarter mile from the lake's outlet, and a wilderness permit with bear canister is required for overnight stays. Many hikers reach Thousand Island Lake as one of the highlights of a longer trek, since it sits squarely on the John Muir Trail south of Donohue Pass. If you want to string it together with Garnet Lake, Shadow Lake, and the climb over the pass from Yosemite, our John Muir Trail section hike from Tuolumne Meadows to Reds Meadow walks the entire corridor and includes this lake.
Best Time to Go
The hiking window runs from roughly July through early October. In a normal year, snow lingers on the passes and shaded slopes into July, and mosquitoes can be fierce in the meadows during the first weeks after melt-out. Late August and September usually deliver the best balance of stable weather, fewer bugs, and crisp light. Wildflowers peak in midsummer, while September brings golden willows along the riverbanks and the first dusting of snow on Banner Peak.
What to Bring
This is high, exposed terrain, so pack accordingly. Afternoon thunderstorms are common in July and August, and the High Trail in particular has little cover. Bring a sun layer, a rain shell, plenty of water capacity, and a bear canister if you are staying overnight. Trekking poles help on the long descent. And bring a real camera, because the sunrise over Banner Peak earns every mile you walk to get there.
