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 Tasman Glacier between 1971 and 1993. Rapid melting began in the late 1960s in a few isolated melt ponds in the centre and in a small elongated lakelet at the eastern lateral moraine. These ponds and lakes grew rapidly in size during the 1970s and coalesced to form a large melt lake by about 1990. This melting has led to a disintegration of the entire terminus region, and now occurs as far as 3 km upstream from the old terminus. The main front of the glacier has retreated c. 1.5 km since 1982. The breaking up of the glacier has been accelerated by the onset of iceberg calving-a process which probably started in 1991. The icebergs can have volumes of several millions of cubic metres before they break up into smaller ice masses that melt slowly during the summer. A temperature survey has shown that the melt lake is almost isothermal (0.3-0.5 degrees C). A poorly understood convection mechanism prevents 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 between 150 and 200 m over the large (almost 2 km(2)) area now occupied by the 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 bathymetric surveys. The present lake level stands at 727 m a.s.l. The surveys demonstrate how the terminus region of the largest New Zealand glacier has disintegrated over the past 22 years.