⛰️ The Largest Natural Freshwater Lake West of the Mississippi — And an Island Full of Wild Horses — Big Arm State Park on Flathead Lake, Montana, part of Flathead Lake State Park system, 197-square-mile glacial lake, largest natural freshwater lake west of the Mississippi, 41 campsites + 3 yurts, boat launch to Wild Horse Island (2,100 acres, bighorn sheep, wild horses, bald eagles), Mission Mountains views, Flathead Indian Reservation, pebble beaches, ponderosa pine — Lake County, MT
Flathead Lake covers 197 square miles of glacial water in northwestern Montana — the largest natural freshwater lake west of the Mississippi. The Mission Mountains rise to 10,000 feet on the east shore. The Salish Mountains wall off the west. And in the middle of the lake sits an island with wild horses.
Big Arm State Park sits on the southwest shore, where a bay reaches into the hills. Pebble beaches under ponderosa pines. A campground with mountain views. And a boat launch that’s the primary gateway to Wild Horse Island — 2,100 acres of roadless wilderness where bighorn sheep outnumber the horses that gave it its name.
What to See
| Feature | Details |
|---|---|
| Flathead Lake | 197 square miles. Up to 370 feet deep. Water clarity of 35+ feet. Glacially carved during the last Ice Age. Fed by the Flathead River from Glacier National Park. The lake is so large it has its own weather patterns |
| Wild Horse Island | 2,100-acre island accessible only by boat. Day use only. Bighorn sheep, mule deer, bald eagles, songbirds — and a small band of wild horses descended from Salish-Kootenai stock. Hiking trails across open grassland and Douglas fir forest |
| Campground | 41 sites including tent-only and RV sites (max 40 feet). 3 yurts (1 ADA-accessible). Flush toilets, coin showers. Bear-resistant storage. Open April–October. First-come or reservable |
| Mission Mountains | The Mission Range rises directly across the lake — snow-capped peaks to 10,000 feet. The sunrise over the Missions from Big Arm is one of the finest views in Montana |
| Boat Launch | Concrete ramp with lake access. The primary launch point for Wild Horse Island trips. Also used for fishing, sailing, and paddling. AIS inspection required for all watercraft |
Wild Horse Island
| Feature | Details |
|---|---|
| The Horses | A small band of wild horses — descended from horses the Salish and Kootenai peoples swam to the island to protect from rival tribes. The horses remain, though the herd is managed at small numbers |
| Bighorn Sheep | The island’s most visible wildlife. A transplanted herd of Rocky Mountain bighorn sheep thrives on the open grasslands. Easier to spot here than almost anywhere in Montana |
| Access | Boat-access only. No public ferry. Private boat shuttles operate from Big Arm in summer. Or bring your own kayak — it’s about a 1-mile paddle from the nearest shore, but lake conditions can change fast |
| Rules | Day use only. No camping. No pets. No fires. Pack everything out. The island is managed as wilderness within a state park |
Best Time to Visit
| Season | Best For |
|---|---|
| Summer (Jun–Aug) | ☀️ Lake at its warmest. Best Wild Horse Island conditions. Full campground. Cherry season along the east shore. Long days and mountain sunsets |
| Fall (Sep–Oct) | 🍂 Larch turning gold in the mountains. Quieter campground. Bighorn sheep in rut on the island. Clear skies and cool nights |
| Spring (Apr–May) | Campground opening. Lake still cold. Wildflowers on the island. Bald eagles nesting. Snow on the Missions |
| Winter (Nov–Mar) | Campground closed. Lake doesn’t freeze (too deep). Eagle watching. The Missions in full winter white |
Frequently Asked Questions
How do I get to Wild Horse Island?
By boat only — there’s no public ferry. Private boat shuttle services operate from Big Arm during summer. You can also paddle (kayak or canoe), but be aware — Flathead Lake can develop serious waves with little warning. Check conditions before crossing.
Is this on tribal land?
Yes — Big Arm is within the Flathead Indian Reservation of the Confederated Salish and Kootenai Tribes. Fishing may require a tribal recreation permit in addition to a Montana fishing license. Check current CSKT requirements before your visit.
Keep exploring: The closest neighbors are Big Arm State Park (a short drive away) and Wild Horse Island State Park (a short drive away).
⛰️ 197 Square Miles of Glacial Water. An Island Full of Wild Horses.
The largest natural freshwater lake west of the Mississippi. Mission Mountains rising to 10,000 feet. A 2,100-acre island with bighorn sheep and wild horses. And a campground with yurts on the shore.
🐾 Dogs & Pets at Big Arm State Park
- Pets allowed? statewide rule
- Pets are allowed at most Montana state parks but must be leashed and not left unattended.
- Leash rule statewide rule
- Leash no longer than 8 feet required, unless otherwise posted.
- Beaches & swim areas statewide rule
- Pets are prohibited from swimming beaches and posted restricted areas.
- Statewide exceptions statewide rule
- Pets prohibited at Frenchtown Pond and Wild Horse Island; kennel service offered during Lewis and Clark Caverns tours; Spring Meadow Lake no pets April 16 - October 14; Lake Elmo has a fenced dog park.
Are dogs allowed at Big Arm State Park?
Pets are allowed at most Montana state parks but must be leashed and not left unattended. Individual parks can post stricter rules — check signage on arrival.
Rules verified July 2026 via fwp.mt.gov. More: Montana state parks guide · Dog rules in all 50 states compared
Park Location
Plan your gear for Big Arm State Park
From our editorial guides, matched to this park’s activities:
