Kazuhisa A Chikita

Department of Natural History Sciences, Faculty of Science, Hokkaido University,

Sapporo, 060-0810,Japan

Many supraglacial lakes have appeared and expanded in the Himalayas since the 1950s by glacial retreat, probably due to global warming after the Little Ice Age. Some of these lakes have produced glacial lake outburst flood (GLOF) events since the 1960s, which have occurred, on average, once every three years somewhere in the Himalayas. The three glacial lakes, Tsho Rolpa (water level 4,580m a.s.l.) and Imja (5,009m a.s.l.) in the Nepal Himalayas, and Lugge (4,539m a.s.l.) in the Bhutan Himalayas, were typical supraglacial lakes in the 1950s to 1960s, but, at present, are moraine-dammed by the subsequent horizontal expansion, enough to touch the side moraine. The lakes were investigated from the hydrological and hydrodynamic viewpoints by field observations from 1995 to 1997, and in 2001 and 2002. In particular, Tsho Rolpa has the highest potentiality of GLOF, since the lake water directly contacts the end moraine, in addition to sufficient water pressure (maximum depth, 131m). Hence, the hydrodynamics of the lake were explored in the pre-monsoon season of 1996 by mooring current meters, temperature loggers and turbidimeters under water, and by obtaining vertical profiles of water turbidity and temperature. As a result, a dynamic model of the lake basin expansion, related to calving at the glacier terminus, was proposed.

A comparison of the thermal structure between Tsho Rolpa, Imja, and Lugge shows a definite difference in the lake hydrodynamics associated with the lake expansion rate. The end-moraine and the dead-ice zone of Imja are 10-25m higher than the lake level. The topographic screening of the end moraine on valley winds, commonly blowing along the elongated lakes, tends to decrease wind velocity near the lake surface, which weakens vertical thermal circulations inducing the ice melt below the lake bottom. The characteristic thermal structure of Imja Lake was probably produced by such a screening

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