The Tungnaá River in Iceland presents a striking, milky-white or sometimes turquoise appearance as it flows through the rugged highlands. This visual characteristic is a direct result of the river’s unique geological origin and its connection to Iceland’s vast ice caps. The opaque water, often described as glacial milk, is caused by the powerful grinding action of the glacier that feeds the river.
Geographical Context of the Tungnaá
The Tungnaá is a significant river system located in the southern Icelandic Highlands, a remote volcanic plateau. The river begins its journey near the western edge of Vatnajökull, the largest glacier in Europe. It flows generally southwesterly before turning northwest, eventually joining the Þjórsá River, Iceland’s longest river.
Historically, the river carried substantial glacial sediments, but its course has been modified by hydroelectric development. Several power stations, including Hrauneyjafossvirkjun and Sigölduvirkjun, are built along its path, resulting in reservoirs like Sigöldulón. The river’s total length is approximately 129 kilometers.
Glacial Flour: The Source of the White Color
The distinctive, cloudy appearance of the Tungnaá is caused by material known as glacial flour, or rock flour. This substance is composed of extremely fine-grained, silt-sized particles of rock that remain suspended within the water column. Glacial flour is essentially pulverized rock, and its presence turns the water opaque, giving it the characteristic milky look.
The particles are typically silicate minerals, ground down to a size that prevents them from quickly settling. When sunlight penetrates the water, these microscopic fragments scatter the light in all directions, similar to the Tyndall effect. This strong scattering causes the water to appear bright white or sometimes a vibrant turquoise, depending on the mineral composition and concentration of the suspended load.
The Mechanics of Rock Grinding
The formation of glacial flour is a continuous process driven by the immense geological power of the ice mass. The Vatnajökull ice cap moves slowly under its own enormous weight, causing the ice to grind against the bedrock beneath it. This process is known as glacial erosion.
As the glacier slides, rock fragments frozen into the base of the ice act like sandpaper on a massive scale. This abrasive action mechanically weathers the solid rock, pulverizing it into the ultra-fine silt that becomes glacial flour. The pressure and friction created by millions of tons of ice reduce the hardest volcanic rocks into a powder measured in micrometers, which is then flushed out by the melting ice.
Maintaining the Milky Suspension
The fine glacial flour must remain suspended for the river to maintain its white color over long distances. The Tungnaá achieves this through its turbulent flow characteristics and steep gradient in the rugged highlands. The river is classified as high-volume, carrying significant water from the melting glacier, which contributes to its high velocity.
This combination of high velocity and turbulence provides the necessary energy to keep the tiny rock particles from settling. The water’s constant movement prevents the force of gravity from pulling the silt-sized particles to the bottom, ensuring they stay distributed throughout the water column. If the river were to become slower or less turbulent, such as when entering a reservoir, the particles would settle out, and the water would clear, as observed in sections modified by hydroelectric projects.