Bear Lake, situated on the border of Utah and Idaho, is nicknamed the “Caribbean of the Rockies” due to its intense, opaque turquoise or azure color. This deep, milky blue prompts a fundamental question about the lake’s composition. The unique color of Bear Lake is not a simple reflection of the sky, but the direct result of a specific mineral interacting with sunlight.
The Mineral That Makes It Blue
The vibrant hue of the lake is caused by the presence of extremely fine, suspended particles of Calcium Carbonate. This mineral is common in geological settings, but in Bear Lake, it primarily takes the form of aragonite, a specific crystal structure of calcium carbonate. These microscopic particles are often referred to as “marl” when they settle on the lake bottom, but while suspended, they are the source of the water’s color.
These aragonite particles are minuscule, often sub-micron in size, which allows them to remain dispersed throughout the water column instead of quickly sinking. The sheer quantity of these suspended solids is what differentiates Bear Lake from other clear lakes. The white color of the mineral itself acts like a vast cloud of tiny mirrors beneath the surface, highly efficient at reflecting and scattering specific wavelengths of light.
The Geological Origin of the Minerals
The source of this widespread calcium carbonate is the surrounding geology of the Bear Lake basin. The lake is a deep, ancient body of water, formed by tectonic movement along a fault line. The mountains bordering the lake, particularly the Bear River Range, are composed of Paleozoic carbonate rocks, essentially limestone.
Inflowing water from springs and streams moves through this limestone bedrock, dissolving calcium and bicarbonate ions. This process saturates the lake with the chemical precursors for calcium carbonate. Bear Lake’s unique, highly alkaline chemistry, which includes a high ratio of magnesium to calcium, favors the precipitation of aragonite over other forms of calcium carbonate.
When conditions are right, such as during warmer summer months when the water warms and photosynthesis by phytoplankton consumes dissolved carbon dioxide, the aragonite chemically precipitates out of the water. This process, sometimes called a “whitening event,” generates the fresh supply of microscopic aragonite crystals that keep the water perpetually cloudy and vividly colored.
The Physics of Light and Color Display
The intense turquoise is ultimately a visual phenomenon based on light scattering. While deep, clear water naturally appears blue due to Rayleigh scattering—where water molecules scatter the sun’s shorter blue wavelengths—Bear Lake’s color is amplified far beyond this natural effect. The thousands of suspended aragonite particles act as much larger scattering centers than water molecules alone.
The size of the aragonite particles is perfect for a process known as Mie scattering, which occurs when particles are roughly the same size as the visible light wavelengths. These particles scatter all visible light but are particularly effective at amplifying the scattering of the shorter, blue and green wavelengths. This strong scattering of blue light in every direction gives the water its intense, milky, and opaque turquoise appearance.