Sir, A supraglacial extension of an ice-dammed lake, Tunsbergdalsbreen, Norway: comments on Dr P. J. Howarth’s paper
Reference LiestølLiestøl (1956) and others (Reference LeopoldLeopold and others, 1964) have pointed out that the potential energy of water in a drainage system is largely converted into heat. In the case of Brimkjelen, the lake described by Reference HowarthHowarth (1968), if we take the volume as 107 m3, the drop from the lake to the snout as 300 m, and the distance as 4 km (Reference KickKick, 1966), then the full drainage of the lake could provide enough energy to melt a tunnel 4 km long and 24 m2 in cross-section. The settling of an irregular ice mass on to an uneven floor would not be likely to cut off all drainage at once. Instead, flow, and thus melting, would be concentrated along a few paths. Even though not all of the available energy would be used for this purpose, melting should be sufficient to keep a tunnel open against slow ice movement. In the absence of very rapid ice movement the tunnel would thus remain open as long as the supply of water lasts, that is, until the lake is either empty or frozen solid. (Obviously this does not apply if drainage is initiated through crevasses well above lake-floor level.) It would therefore be interesting to see whether there is any correlation between the regimes of such lakes and the flow characteristics of the associated glaciers.
19 November 1968
