The lakes within Banff National Park, such as Lake Louise and Moraine Lake, exhibit a vivid turquoise color. This hue, which attracts millions of visitors, is not the result of a dye or unusual chemical composition but stems from a unique interaction between geology, water, and sunlight. This optical effect is rooted in the specific source of the water and the finely ground material it carries.
The Origin: Glacial Meltwater
The source of the colored water is the high-altitude glaciers that cap the Canadian Rockies. As ice flows down the mountain slopes, it scrapes and scours the underlying bedrock. This erosive process carves out the valleys that the lakes now occupy. The friction pulverizes the bedrock into a fine, powder-like substance. Glacial meltwater, which increases during summer, carries this pulverized material into the lakes, initiating the color process.
The Key Ingredient: Rock Flour
The fine material suspended in the water is known as “rock flour” or glacial silt, and it is the component for the turquoise coloration. Rock flour consists of silt- and clay-sized particles that are too light to settle quickly to the lakebed. Instead, they remain suspended throughout the water column, creating a milky texture. The composition is derived from sedimentary rocks common in the region, including limestone, dolostone, quartz, and feldspar. These particles are less than a few micrometers in diameter, and their fineness allows them to stay in suspension for long periods before eventually settling out. This concentration of suspended mineral particles provides the medium for manipulating light.
The Mechanism: Scattering Blue Light
The perceived blue color of the lakes results from the way these fine particles interact with incoming sunlight. When white sunlight penetrates the water, the suspended rock flour particles selectively scatter different wavelengths of light. Light is made up of a spectrum of colors, where blue and green have shorter wavelengths, and red and yellow have longer ones. The size of the glacial silt is efficient at scattering the shorter, higher-energy blue and green wavelengths back toward the surface and the observer’s eye, while the water simultaneously absorbs the longer wavelengths, such as red, orange, and yellow. This combination—water absorbing warmer colors and rock flour scattering cooler colors—produces the vivid turquoise hue that defines Banff’s lakes. The intensity of the color is directly related to the concentration of the rock flour and the angle of the sun, making the lakes appear most brilliant on clear, sunny days in mid-summer when glacial melt is at its peak.