Have you ever been to North Cascades National Park in Washington State and stared out at their majestic lakes and wondered…how are the heck are they so impossibly blue? The answer is simple, glacial flour.
Glacial flour, also known as rock flour or glacial silt, is micro sediment produced by glaciers grinding on the underlying bedrock. How small are these tiny quartz and feldspar particles? Glacial flour grains range between 2 to 65 microns in diameter. To provide some perspective, table salt usually measures 100 microns.
Due the very fine particle size, glacial flour remains suspended in glacial meltwater, producing the characteristic cloudy or milky appearance. This phenomenon is known as “glacial milk.” When exposed to sunlight, the fine particles of glacial milk scatter the light and reflect mainly green and blue wavelengths, producing the striking turquoise blue hues like you see in North Cascades.
Why so blue?
A common question that rangers are asked here in the North Cascades about the stunning hue of the impressive Diablo Lake. So why is it so blue? Or green? Or is it aquamarine?
The answer is simple: glimmering glacial flour is poured in every season by rangers!
Almost but not quite…Diablo Lake, and many of the other lakes in the park complex is fed by hundreds of streams with origins in the steep mountains and glaciers. These streams carry finely ground rock particles called glacial flour. These particles cloud the water and cause the water to have the famous blue hue we know and love. Thunder Creek, for example, carries the drainage from over fifty glaciers before flowing into the Diablo Lake. It is these streams that deserve the applause, not rangers (well…at least not for this reason).
