Why Lake Brienz Looks So Turquoise

Lake Brienz isn’t “painted” turquoise by algae or minerals dissolved in the water. The colour comes from something far more Alpine: tiny particles of rock, ground down by glaciers, carried into the lake by mountain rivers, and held in suspension long enough to turn sunlight into that signature blue-green glow.

Below is the simple story of how it happens, and why the shade can look different depending on season, weather, and even the time of day.

It starts in the glaciers: rock ground into powder

High in the Bernese Alps, glaciers move. And as they move, they grind bedrock beneath them like sandpaper. That process creates an extremely fine sediment often called rock flour (or glacial flour): microscopic mineral particles that can be as fine as silt.

When meltwater picks up those particles, it carries them downhill into streams and rivers. Once they reach a lake, they don’t immediately drop to the bottom. Because they’re so small, they can stay suspended for a long time.

This is the key ingredient for turquoise.

Two rivers “feed” the colour: the Aare and the Lütschine

Lake Brienz has two main inflows: the Aare and the Lütschine. Both originate in high alpine catchments with glacial influence, which means they deliver not just water, but also a lot of fine sediment into the lake.

Researchers studying Lake Brienz describe its sediment record as dominated by fine-grained material, supplied by these rivers. In other words: the lake is constantly receiving a very fine “dusting” of mineral particles that settles slowly over time.

That slow settling is part of why the colour effect can be so persistent and so intense when the light is right.

The physics bit: why our eyes see turquoise

Once those microscopic particles are suspended in the water, they interact with sunlight.

Here’s the intuitive version:

1. Sunlight contains all colours.
2. The tiny suspended particles scatter light.
3. Shorter wavelengths (blue and green) scatter more strongly than longer wavelengths (like red).
4. The scattered blue-green light is what bounces back to your eyes.

So the lake can look like it’s glowing from within, especially when the surface is calm and the sun is low enough to create that clean, angled light.

It’s the same reason many glacier-fed lakes around the world show similar milky-turquoise tones: suspended rock flour plus sunlight equals magic.

Why the colour changes through the year (and even during the day)

If you’ve seen Lake Brienz on multiple visits, you already know the turquoise has moods. That’s normal, and it’s exactly what you’d expect from a lake whose colour is driven by sediment and light.

Seasonal shift: meltwater changes the “mix”

In periods with more snowmelt and glacier melt, rivers can carry more fine particles into the lake. That can make the turquoise look brighter and sometimes slightly “milkier.”

In colder periods, when meltwater input is lower, the lake can look deeper and darker, more blue-green than pastel. The turquoise is still there, but it can be subtler unless the light is strong.

Sunlight: more light, more colour

Turquoise is a visual effect. The more sunlight hits the water, the more obvious the scattering becomes. That’s why the colour can look completely different between:

• early morning vs midday
• direct sun vs flat cloud
• winter low-angle light vs high summer sun

Wind and surface texture: calm water shows it best

Choppy water breaks up reflections and makes it harder to “read” the colour. On calm days (and especially on winter-mirror days), the turquoise can look cleaner, richer, and more three-dimensional.

Quick questions people ask

Is the turquoise a sign of pollution?

No. The colour is primarily linked to natural, fine mineral sediment delivered by glacially influenced rivers.

Is it algae?

Lake Brienz is known as a nutrient-poor lake, which generally means less algae than many other lakes. That helps the mineral-sediment colour show through rather than being masked by green blooms.

Why does Lake Thun often look different?

Different catchments, different sediment loads, different biology. Two nearby lakes can behave very differently depending on what flows into them and how nutrients circulate.

Further reading

• Lake Brienz physical characteristics, trophic state (ultra-oligotrophic), residence time, and the Aare and Lütschine as the major water and sediment suppliers: access.archive-ouverte.unige.ch
• Fine-grained (silt-sized) sediment in Lake Brienz and its link to fluctuating sediment input from the Aare and Lütschine: (limnogeology.ethz.ch)
• General explanation of glacial flour being carried into lakes, remaining suspended, and scattering sunlight to produce a cloudy turquoise appearance: (National Park Service)
• NASA explainer on glaciers producing fine rock particles that absorb and scatter sunlight to create blue-green lake colour: (NASA Science)
• Local destination explanation linking Lake Brienz turquoise colour to fine glacial sediment from the Aare and Lütschine and noting seasonal intensity: (interlaken.ch)