tarn

A tarn is a mountain lake or pool that forms in a cirque, a bowl-shaped depression carved into bedrock by a glacier. The word tarn derives from Old Norse and entered English through Scottish usage, and it is commonly used to describe small, high-elevation lakes in glaciated regions.

Formation and morphology: Tarns form when a glacier erodes a hollow in a mountainside, creating a cirque. As the glacier retreats, meltwater fills the hollow, yielding a body of water that is often fed by snowmelt and alpine streams. Tarns are typically surrounded by steep, rocky walls and may have steep, rocky shores. Their depth and clarity can vary, and some retain glacially fine sediment or bedrock debris at the bottom.

Hydrology and geology: The basins are usually bedrock with impermeable sides, limiting exchange with surrounding groundwater. Because of cold temperatures and limited nutrient input, tarns tend to be oligotrophic, with clear water and low biological productivity. Algal growth is generally sparse, and the littoral zone is restricted to sheltered shorelines.

Ecology and environmental considerations: Tarns host specialized aquatic invertebrates and cold-water-adapted plants along their margins. They are sensitive to nutrient enrichment, pollution, and climate change, which can alter water temperature, chemistry, and water levels. Reduced snowpack and glacial retreat can lead to changes in tarn size and ecological balance.

Human use and distribution: Tarns are valued for recreation and scenic beauty and are common in the Alps, Pyrenees, Scottish Highlands, and the North American Rockies. They often lie within protected landscapes and are accessed by hiking trails and viewpoints. See also glacial landforms and alpine lakes.