DOWNWASTING OF THE TASMAN GLACIER, SOUTH ISLAND, NEW-ZEALAND - CHANGES IN THE TERMINUS REGION BETWEEN 1971 AND 1993

Downwasting has altered the morphology of the terminus region of the T asman Glacier between 1971 and 1993. Rapid melting began in the late 1 960s in a few isolated melt ponds in the centre and in a small elongat ed lakelet at the eastern lateral moraine. These ponds and lakes grew rapidly in size during the 1970s and coalesced to form a large melt la ke by about 1990. This melting has led to a disintegration of the enti re terminus region, and now occurs as far as 3 km upstream from the ol d terminus. The main front of the glacier has retreated c. 1.5 km sinc e 1982. The breaking up of the glacier has been accelerated by the ons et of iceberg calving-a process which probably started in 1991. The ic ebergs can have volumes of several millions of cubic metres before the y break up into smaller ice masses that melt slowly during the summer. A temperature survey has shown that the melt lake is almost isotherma l (0.3-0.5 degrees C). A poorly understood convection mechanism preven ts suspended silt from settling and causes the uniform grey colour of the lake (here called ''Tasman Lake''). Gravity surveys in 1971/72 and in 1982 revealed that the average thickness of the glacier was betwee n 150 and 200 m over the large (almost 2 km(2)) area now occupied by t he melt lake. The bottom level of the glacier was close to 600 m a.s.l .; this level has been confirmed by recent radar soundings and bathyme tric surveys. The present lake level stands at 727 m a.s.l. The survey s demonstrate how the terminus region of the largest New Zealand glaci er has disintegrated over the past 22 years.