Iceland has many freshwater lakes, in diverse geomorphological settings. The origins of these lakes differ. Skorradalsvatn and Lagarfljót are two examples of lakes formed due to glacier erosion during the Ice Age. Ljósavatn and Hreðavatn were formed during volcanic eruptions when lava blocked a depression or valley. Þingvallavatn and Skjálftavatn in Kelduhverfi came into being due to tectonics or land subsidence; the latter lake formed in 1976–1977 during a volcanic episode at the Krafla volcanic system. Hraunsvatn in Öxnadalur and Flóðið in Vatnsdalur are products of major landslides. Finally, glacial lakes and lagoons will become ever more prominent in the vicinity of glaciers as these glaciers melt at an increasingly rapid rate in a warming climate. Human-created reservoirs, constructed in connection with hydroelectric dam projects, are a new type of lake in addition to these natural formations.

Most lakes in Iceland are rather small. Lake Þingvallavatn was for a long time Iceland’s largest lake, at 83 km2. With the completion of the Vatnsfell Power Station in 2001, Lake Þórisvatn grew to 86 km2. Lake Öskjuvatn, which formed during a volcanic episode in Dyngjufjöll and Askja in 1874–1875, was long believed to be Iceland’s deepest lake. It is 220 m deep. Recent measurements indicate that the glacial lagoon Jökulsárlón is in fact deeper, or 248 m deep. Despite its popularity as a tourist destination, Jökulsárlón is a very young lake and still in the process of forming. It first appeared in 1933, with the retreat of the Breiðamerkurjökull glacier, which is an outlet glacier of the much larger Vatnajökull ice cap. Jökulsárlón continues to grow as the glacier retreats and meltwater fills the deep rut that the glacier has dug into the sand beneath.