HUSS M., BAUDER A., WERDER M., FUNK M., HOCK R. Glacier-dammed lake outburst events of Gornersee, Switzerland. Journal of Glaciology, 2007, Vol. 53, N° 181, p. 189-200.

Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie (VAW), Eidgenössische Technische Hochschule, ETH-Zentrum, CH-8092 Zürich, Switzerland. mhuss@vaw.baug.ethz.ch
Department of Earth Sciences, Uppsala University, SE-752 36 Uppsala, Sweden.

The lake usually starts to fill in May and drains annually between June and August (Bezinge et al., 1973). Each year 1-5 millions m3 of meltwater are impounded by the lake. The duration of the lake drainage is 2-7 days. The peak discharges during the outburst events measured at the glacier terminus reach 20-50m3/s, of which 40-75% is lake water (as calculated in this study). In the first half of the 20th century, flood intensities of more than 100 m3/s were reported, regularly causing severe damage in the valley of Zermatt (Raymond et al., 2003).

Significant drainage events in every year since 1970, except for 1984, 1991 and 1995 have been identified. Between 1950 and 2005 a shift of about 2 months (70 days) has been observed, moving the expected date of the event from late August to late June. In contrast, the temporal evolution of drainage volume does not show a uniform trend. In addition to the year-to-year variability, long-term fluctuations of drainage volumes also occurred.

A maximum in drainage volumes was observed in the early 1970s and a significant minimum occurred in the 1980s. In the 1990s the data reveal a more variable pattern of flood magnitude than before, with either quite large or small floods. There is no correlation between drainage volume and the timing of the lake outburst. The volume of the lake is subject to substantial long-term changes which are attributed to fluctuations in glacier geometry. The glacier retreat in 1995-2005 favored a rapid enlargement of the ice-marginal basin, mainly due to calving.

To investigate the possible influence of an earlier start of the melting season and thus earlier filling of the lake, additional model simulations were conducted. They show a slight trend towards more melting in late spring and early summer. However, this change can only explain shifts in timing of a few days.

Extensive field measurements around the drainage events of 2004 and 2005 provide the data for an integrative assessment of the outburst of Gornersee. Two different drainage mechanisms were detected, a temporary flotation of the ice dam in 2004 and classical channel enlargement in 2005 after a leakage over 8 days. So, even for an individual system, a variety of different processes are involved which alter the character of the drainage with varying degrees of importance.

Analysis of the lake drainage events of Gornersee from 1950-2005 indicates trends in lake outburst timing and large variations in the drainage volume. Several processes may be the cause of these findings. Rapid shrinkage of ice volume and glacier extent took place in recent decades due to climate change. It is likely that these modifications of glacier geometry influence lake drainage.

The authors suggest that the catchment area of the lake has increased during the past three decades, causing an earlier filling of Gornersee. A contribution of this effect on the observed trend in the outburst timing is likely, but its relative importance is difficult to estimate. The changes in glacier geometry around the lake are considerable. Although we do not know the exact position of the seal for all past glacier geometries, a shift may have occurred, influencing the timing of the outburst event.

The authors suspect the geometry change in the lake basin is not the sole reason for the variations in drainage volume. In particular, the interannual fluctuations may be caused by different initiation mechanisms leading to the lake draining subglacially, either partially filled (as observed in 2005) or after an antecedent supraglacial outflow (2004).