The Ultimate Big Day Hike Guide

I will fully admit that the first time I did this one, I underestimated it so badly I was still hiking out well after dark with a headlamp I'd thrown in my pack as an afterthought, thank god. Thousand Island Lake is not a casual outing dressed up with a dramatic name — it's a genuinely big day, even for people who hike a lot, and it deserves way more respect than "day hike" usually implies. If you're going to do one truly ambitious day trip out of Mammoth this summer, this is the one, and here's everything I've learned making it harder on myself than it needed to be.

Quick Stats

• Distance: 14.9 to 19.9 miles round trip depending on route (details below)
• Elevation gain: 2,400 to 3,900 feet depending on route
• Difficulty: Hard — this is a full mountain day, not a stroll with a nice name
• Trailhead elevation: About 8,340 ft | Thousand Island Lake elevation: About 9,833 ft
• Trailhead: Agnew Meadows, off Reds Meadow Road
• Fishing: Yes, brook and rainbow trout, California fishing license required
• Overnight permit: Only required if you're camping — day hiking needs no permit
• Shuttle required: Almost always, and this is the part that will make or break your day

Why This One's Different

Thousand Island Lake sits at the base of Banner Peak, and it earns every bit of its reputation as one of the iconic views in the Ansel Adams Wilderness. The lake is genuinely studded with small rock islands, and on a still morning the reflection of Banner Peak across the water is the kind of thing that makes you understand why people fly in from other states just to see it. I've done this hike more than once, and I still get a little stunned every time I come around the final bend and the lake actually opens up in front of me. It's a lot of miles to earn that view, but it earns it back.

Getting There: Shuttle vs. Driving Yourself

This is genuinely the part that trips people up more than the mileage does, so let's get it sorted first.

The mandatory shuttle window. Agnew Meadows sits on Reds Meadow Road, which runs the same shuttle system as Devils Postpile and Rainbow Falls, and during operating hours nearly everyone has to ride it in. Agnew Meadows is Shuttle Stop 1, so it's actually one of the earlier stops on the route, which is a small mercy on a big-mileage day.

2026 shuttle schedule (same system serving the whole Reds Meadow Valley):

• Season: Late June through September, road and weather permitting
• Thursdays: 7:30 a.m.–7 p.m., roughly every two hours
• Friday–Sunday: 7:30–9:00 a.m. every 45 minutes, 9:00–10:00 a.m. every 30 minutes, 10:00 a.m.–4:20 p.m. every 20 minutes, 4:45–6:15 p.m. every 45 minutes
• Last bus out of the valley: 7:00 p.m. sharp

If you can start early, drive yourself. Vehicles are allowed to drive into the valley outside shuttle operating hours — specifically before 7:00 a.m. or after 7:00 p.m. Given how long this hike is, arriving before 7 a.m. and parking at Agnew Meadows directly is honestly the smarter move if you can manage it, since it buys you real daylight instead of losing your first hour to the shuttle line. Just know you'll still need to be aware of the shuttle's operating hours for your return, since exiting mid-afternoon on a Friday through Sunday means you're back in shuttle territory unless you're out before 7 a.m. or planning to hike out after 7 p.m.

2026-specific heads up: Reds Meadow Road has been under active reconstruction, and access schedules have shifted year to year, including days where the road is fully closed to all traffic, on foot included, outside of weekend windows. This is the single biggest variable on this whole hike, and I'd be doing you a disservice pretending I can guarantee it'll be identical by the time you go. Check current conditions at the Mammoth Lakes Welcome Center or nps.gov/depo before you commit to a date, and build in a backup day if you can.

Route Options

There are three real ways to do this, and they're different enough hikes that it's worth picking on purpose rather than by accident.

1. River Trail (out-and-back) — the moderate option About 14.9 miles round trip, roughly 2,400 feet of gain. This route follows the San Joaquin River for most of the way, which means more shade, more reliable water, and a gentler overall grade than the alternatives. If you want the most forgiving version of this hike, this is it.

2. PCT High Trail (out-and-back) — the scenic option About 16.2–16.4 miles round trip, closer to 2,900 feet of gain. This trail climbs higher and stays higher, tracing a genuinely stunning ridgeline with sweeping views over the San Joaquin River valley the whole way. It's also the most exposed route on this list by a wide margin — more on that below.

3. The loop — River Trail one way, High Trail the other This is the one I'd actually recommend if you're capable of the extra mileage, somewhere around 17–18 miles total. You get the shade and water reliability of the River Trail on one leg, and the views of the High Trail on the other, without retracing your own steps for eight-plus miles like a person who ran out of imagination. I did the out-and-back on my first attempt and regretted not looping it almost immediately.

Trail Exposure — Read This Before You Commit

The PCT High Trail is exposed for nearly its entire length. There is real, sustained sun with very little tree cover once you're up on the ridge, and if you're hiking it in the middle of summer, that's a lot of hours in direct sun at altitude. I made the mistake of treating it like any other hike and paid for it in a genuinely bad sunburn and a headache that felt suspiciously like the beginning of altitude sickness by the time I reached the lake.

The River Trail, by contrast, spends a good amount of its lower and middle sections under real tree cover, which is a meaningful difference on a hike this long. If you're doing the loop, I'd personally do the High Trail early in the day while the sun is lower and finish on the shadier River Trail in the afternoon heat, though plenty of people prefer it the other way for the views on the way out. Either way, this is not a hike where you can get away with underestimating sun exposure just because part of it looks forested on a map.

Water

The River Trail is the reliable option here — you're following the San Joaquin River basically the whole way, so water access and filtering opportunities are frequent. The High Trail is drier by nature, though it does cross a number of downhill-flowing streams along the ridge, especially earlier in the season when snowmelt is still feeding them. That said, stream flow on the High Trail can thin out considerably by late summer, so don't assume the same water sources will be flowing in September that were flowing in July. I carry a filter either way and treat every stream as a "maybe," not a guarantee, especially late season.

Fishing at Thousand Island Lake

Bring a rod if you're into it — Thousand Island Lake holds both brook and rainbow trout, and a lot of people make a point of building in extra time at the lake specifically to fish. You'll need a valid California fishing license, and given how popular this lake is, practicing catch and release helps keep it that way for the next person who hikes all this way hoping to do the same thing.

What to Bring for a Hike This Big

This isn't a "throw a water bottle in a tote bag" kind of day, and I say that as someone who has personally underpacked for it and regretted every choice on the way back.

Footwear: Real hiking shoes or boots, broken in well before this hike, not on it. Mileage like this will find every weak point in unfamiliar footwear.

Clothing and sun protection: Sun-protective layers matter more here than on almost any other hike on this list, especially if you're taking the High Trail. Hat, sunglasses, and sunscreen you actually reapply, not just remember owning.

Water and a filter: Bring more than you think you need, and bring a way to refill along the way rather than trying to carry the entire day's supply from the trailhead. This hike is too long to carry it all.

Food: Real calories, not a granola bar and good intentions. This is an all-day effort at elevation, and you will feel the difference if you underfuel it.

Trekking poles: Genuinely useful given the mileage and elevation change, especially by the return leg when your legs start negotiating with you.

Headlamp, every time, no exceptions. Even if you plan a perfectly timed day, big mileage at altitude has a way of running long. I've been caught out later than planned more than once, and the one time I didn't have a headlamp with me is not an experience I'm eager to repeat.

Extra layer and basic emergency items. Given the shuttle cutoff, the remote trailhead, and how long this day genuinely is, I now treat this hike the way I'd treat a much more serious outing: an emergency bivy or space blanket, a basic first aid kit, extra insulating layer, and a way to signal or call for help if something goes wrong. None of this is meant to scare you off the hike — it's meant to make sure that if you turn an ankle eight miles in, or the shuttle situation goes sideways on your way out, you're not relying purely on good luck.

A permit, only if you're staying overnight. Day hiking Thousand Island Lake requires no permit at all. The moment you're camping, that changes, and Ansel Adams Wilderness permits for this area are competitive, so plan well ahead if you want to turn this into an overnight.

If You Want to Go Bigger: The Loop to Ediza, Minaret Lakes, and Iceberg Lake

If Thousand Island Lake on its own isn't enough for you, and for some of you it won't be, there's a genuinely spectacular bigger loop that connects Thousand Island Lake with Ediza Lake, Minaret Lake, and Iceberg Lake via the Shadow Creek Trail and the JMT. It's a serious undertaking, more realistically a two-day trip for most people than a single push, and it deserves its own full write-up rather than a rushed mention here. I'm working on that one separately, so keep an eye out for it if this hike leaves you wanting more — it will.

A Few Things I've Learned the Hard Way

• Don't underestimate the High Trail's exposure just because part of your route looks forested on a map. Sun protection is not optional here.
• Bring a headlamp even if your plan doesn't call for one. Big mileage days run long more often than they run short.
• If you can start before 7 a.m. and drive yourself in, do it. It buys you real daylight on a day that needs every bit of it.
• Check current Reds Meadow Road status close to your trip date, not weeks in advance. Construction schedules shift.
• Treat this like the serious mountain day it is, not a long walk with a nice name. I didn't, the first time, and I paid for it.