A tarn is a distinctive type of mountain lake, pond, or pool found in high-altitude environments that have been shaped by ice. These relatively small, often remote bodies of water are direct evidence of past or present glacial activity and are unique features of alpine landscapes worldwide.
Defining the Tarn
A tarn is a small body of water located specifically within a cirque, which is a steep-walled, bowl-shaped hollow carved out of a mountainside. The cirque, sometimes called a corrie or cwm, creates an amphitheater-like basin where the water collects. Tarns are found at high elevations and are often surrounded on three sides by sheer rock cliffs.
The water is frequently clear and cold, reflecting the sparse vegetation and minimal sediment input from the surrounding slopes. The word “tarn” itself is derived from the Old Norse word tjörn, meaning “pond,” reflecting their modest dimensions compared to larger valley lakes.
Tarns are frequently found in formerly glaciated regions, such as the Rocky Mountains, the Alps, and the Himalayas, serving as lasting reminders of the Ice Age. Their presence indicates where a small, alpine glacier once rested and began its erosive work. The basin holding the water is a primary characteristic, differentiating a tarn from other high-altitude lakes.
The Glacial Process of Formation
The formation of a tarn begins with a small, localized glacier, known as a cirque glacier, occupying a pre-existing hollow on a mountainside near the firn line. As snow accumulates and compacts into ice, the glacier begins to move under its own weight, initiating the process of glacial erosion. This movement is often rotational, which acts like a giant scoop to deepen the depression.
Glacial erosion occurs through two primary mechanisms: plucking and abrasion. Plucking happens when meltwater seeps into cracks in the bedrock, freezes, and expands, pulling away large fragments of rock as the glacier moves. Abrasion involves rock debris embedded in the base of the ice, which grinds against the bedrock, scouring and polishing the cirque floor.
Over thousands of years, this combined action carves out the characteristic deep, bowl-shaped basin of the cirque. The tarn itself forms when the glacial ice melts and the depression fills with water from rain, snowmelt, and runoff. Water retention is often aided by a lip of resistant bedrock or by a moraine, which is a natural dam of unsorted glacial till deposited at the lower edge of the cirque.
Tarn Ecology and Water Characteristics
The water in tarns exhibits unique chemical and physical properties due to their glacial origin and high-altitude location. They are classified as oligotrophic, meaning they are low in biologically useful nutrients such as nitrogen and phosphorus. This nutrient-poor state results in minimal growth of algae and aquatic plants, which contributes to the water’s clarity.
Because they are fed primarily by cold snowmelt and are situated at high elevations, tarn waters maintain low temperatures throughout the year. These cold temperatures allow the water to hold high concentrations of dissolved oxygen, particularly in the deeper layers. This environment is highly selective, supporting only specialized, cold-water organisms.
The aquatic life in a tarn is adapted to these harsh conditions, often including hardy species of plankton, insect larvae, and sometimes cold-water fish like trout or steelhead. Low productivity means the ecosystem supports a limited biomass, but the organisms present are highly adapted to survive the short growing seasons and nutrient scarcity. In some cases, if the tarn is still actively receiving runoff from a nearby glacier, the water may appear bright turquoise due to a suspension of fine, glacially-ground rock particles known as glacial flour.