Tarns

A tarn is a small mountain lake formed in a cirque, a bowl-shaped hollow carved by a glacier. The term is commonly used for lakes at high elevations in mountain regions and is sometimes interchangeable with cirque lake. Tarn formation occurs when a glacier erodes bedrock to create a basin; after the ice retreats, meltwater and precipitation fill the depression, often with steep rock walls surrounding the basin.

Tarns are typically cold, clear, and oligotrophic, with low nutrient levels and limited productivity. They often have little inflow and outflow, or a small stream draining the basin, which helps stabilize the water level. Because they are fed by snowmelt and rain, their water tends to be very cold, and many are stratified in summer.

Distribution and characteristics: Tarns occur worldwide in mountainous regions, including the European Alps, Scottish Highlands, Pyrenees, North American Rockies and Sierra Nevada, the Andes, Himalayas, and New Zealand's Southern Alps. They are usually found above or near the treeline, in glaciated terrain where cirques occur.

Ecology and use: The isolation and nutrient-poor conditions limit complex aquatic communities, but some invertebrates and cold-water species may inhabit tarns. They are sensitive to environmental changes such as climate warming and acid deposition. Tarns are popular targets for hikers and climbers and are often located within national parks or protected landscapes.