Lake McArthur

Pros & cons

PROS

Deepest lake in the park with vivid sapphire color
Rewarding alpine hike with high- and low-level route options

CONS

Access hinges on Lake O’Hara’s permit/lottery system
High-level route involves minor scrambling and fickle alpine weather

Overview

Tucked in a high limestone bowl above the Lake O’Hara area, Lake McArthur is Yoho’s deepest lake (85 m) and famously sapphire blue. It’s a pure wilderness scene: no roads, no facilities—just rock, water, and big peaks like Mount Schaffer and Park Mountain framing the shoreline.

Why it’s special

McArthur’s color is outrageous on sunny days, and its setting is unusual: a karst basin with no visible outlet. Water seeps through cracks in the limestone, and you actually hike uphill to leave the lakeshore—odd but true. It’s long been a photographers’ darling and a classic objective for O’Hara hikers.

Getting there

You reach McArthur from the Lake O’Hara trail network, typically passing the Elizabeth Parker Hut and Schaffer Lake before choosing either the McArthur High Level or Low Level route. Plan roughly 6.3 km and 2–3 hours each way via the O’Hara area, and remember: access to O’Hara itself is controlled by reservation/lottery.

Trail options

High Level: Airy, with a short bit of minor scrambling; most hikers prefer doing the high route uphill for better footing and control.
Low Level: Easier and more sheltered; a common choice for the return.

What you’ll see

Expect a greatest-hits mix of alpine meadows, boulder fields, and wildflowers (moss campion is noted) before the big reveal: McArthur’s deep-blue water set on a windswept plateau. On breezy days the surface chops up; longer exposures can smooth it out for photos.

Wildlife and closures

This is grizzly country. The McArthur Valley has a seasonal closure from May 1 to August 15 for bear activity; afterward, only limited permitted groups are allowed in that valley. Carry bear spray, travel smart, and check current advisories when planning routes in the area.

History & geology

Early Rocky Mountain travelers were praising McArthur as a "sapphire gem" over a century ago. Its karst hydrology—no surface outlet, water draining through limestone—explains both the color-rich clarity and that uphill walk away from shore. Glaciers once calved icebergs into the lake from nearby Mount Biddle.

Best time to visit

Late summer into early fall is prime, after the grizzly closure window. Sunny days make the sapphire tones pop; autumn brings larches and crisp air. Crowds thin compared to O’Hara’s core lakes once you’re on the McArthur trails, but permits still gate overall access.