Lake Tasman is the terminal lake of the Tasman Glacier, New Zealands longest glacier at approximately 29 kilometres in length. As recently as 1973, Lake Tasman did not exist and the glacier abutted against the terminal moraine at the lakes southern end, but in recent years a warming of the local climate has led to glacier retreat and the formation of the terminal lake.
The appearance of a terminal lake has caused a rapid increase in the rate of retreat of the Tasman Glacier, because ice that is in contact with liquid water melts several times faster than ice that is in contact with air or buried under moraine. By April 2008, Lake Tasman was measured to be 7 kilometres long, 2 kilometres wide and an astounding 245 metres deep, and increasing rapidly in length as the Tasman Glacier continued to retreat up-valley. The glacier is expected to retreat a further 9 kilometres over the next decade or two until it terminates at a level above that of the encroaching lake, at which point the rate of melting is expected to drop enough for the glacier to reach a new equilibrium. By this stage Lake Tasman would be 16 kilometres long, more than twice its current size.
Even though these changes are thought to be linked to climate change, winters here are still cold enough in some years for the lake to freeze over for a few weeks. This photograph was taken in July 2001 after a prolonged cold snap had frozen the entire lake surface to a depth of 30 cm or so. A norwester over the preceding day had left the surface dappled with small patches of wind-blown snow, adding to the unusual pattern of interlocking shapes that was already embedded in the ice.