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Glacial Expert Talks Through why Lake Pukaki is so Blue

Glacial Expert Talks Through why Lake Pukaki is so Blue

If you haven’t been to Lake Pukaki, it’s hard to believe it’s actually the colour it is. You’ll be convincing your friends #nofilter for days.

Who better to explain Lake Pukaki’s unique colour than Heather Purdie, senior lecturer in Glaciology at the University of Canterbury. We caught up with her to talk us through it. #ExploreCHC

So why is Lake Pukaki so blue?

The blue (turquoise) colour is due to fine silt particles, or glacial flour, in the water. This is a result of glacial erosion. The silt is so fine it does not settle to the bottom quickly, remaining in suspension in the lake water.

When light hits the surface of the lake, the silt absorbs some of the light (the very short wave lengths like purple and indigo) and the water absorbs the longer wavelengths (like red, orange and yellow). This leaves the blue-green wavelengths to be scattered back to our eyes, so we see the lake as a blue/green colour.

Sorry, glacial flour? What’s that exactly?

Glacial flour is the very fine particles of clay produced by glacial erosion. As the glacier flows down the valley, the ice, which contains many rock fragments, grinds over the valley floor and erodes the rock producing the fine sediment.

Is there a time of year the lake may appear brighter than other times?

The intensity of the blue colour can vary depending on the amount of water input where it comes from. The lake is not just fed from glacial streams but from other streams around the catchment. A lot of fresh water will dilute the concentration silt, so the colour of the lake may not appear as ‘blue’.

Further up the valley in Aoraki Mount Cook National Park is the Tasman Glacier lake where the water is quite grey, because of a very high concentration of silt particles. Here the glacier flows directly into the lake, so the water is dominated by glacial melt water.

You’ve done research into sustainable tourism. What’s the most responsible way people can experience Lake Pukaki and the surrounding area?

Our glaciers are shrinking because of human-induced climate warming. Choose low-carbon transport options where possible and take all your waste with you. Better yet, work to minimise your waste – do your best to look after our Earth.

When you are visiting Lake Pukaki and cast your eyes up the sides of the hills. You’ll see distinctive terraces, called moraines – they mark the previous extent of the glacier. During the peak of the last ice age (around 20,000 years ago), the glacier that formed Lake Pukaki used to flow all the way to the modern-day highway.

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