Jökull


Jökull - 01.12.1967, Síða 58

Jökull - 01.12.1967, Síða 58
to flow to the east directly into the Skaftá river. As the glacier has retreated the pass has become wider ancl deeper and the glacier river there has continuously been moving its source farther into the pass. In 1966 that river had captured all drainage frorn the Skaftárjökull glacier west of Fögrufjöll. No other water drain- ed into Langisjór than rivulets of clear water oozing fronr the stagnant ice at the uppermost terminal moraine. Briefly, the history of Langisjór appeared to G. Kjartansson to have been like this: Some centuries ago and even recurrently since “the warm climate period ended nearly 2500 years ago” the glacier has reached the north-eastern end of Fögrufjöll and blocked the drainage from Langisjór through there, causing a raising of the lake level and the formation of the shorelines. He considers the upper one to be formed by stagnant lake level during a long time perhaps many centuries. We on the other hand consider all changes at Langisjór to have taken place within much shorter time than Ivjartansson maintains in his report. Our arguments are the following: 1) The shorelines at Langisjór are cut in pumice from the Laki eruption of 1783 and therefore must be definitely younger than this eruption. 2) The rapid increase in land at the north-eastern encl of the lake, as testified on maps ancl aerial photographs, has for the most part taken place in the present century. 3) Ut- fall, the outlet, has according to sources been formed in the period 1878—1894. Therefore, we conclude tliat the glacier did not extencl to the slopes of Fögrufjöll until in the latter part of the nineteenth century. Before that Langisjór has either been drained between the glacier and Fögrufjöll or been without surface drainage. Fig. 8 presents a diagram illustrating those changes in the water level of Langisjór we con- sider to have occurred during the last 100 years. It is divided into five stages. Stage I shows the lake level before the lake began to be afíected by the glacier shortly after 1850. The lake level is supposed to have been in 650 m elevation in this stage. II began as the glacier started to aftect the lake level. Glacier rivers began to drain into the lake resulting in a gradual raising of the water level. The glacier also blocked the dramage via the passage be- tween the glacier and Fögrufjöll. This stage cante to an end about 1890 as the glacier extended into the lake ancl the lake level reach- ed its maximum elevation forming the upper shoreline. Stage III began as water started to overflow the rock barrier at the present Utfall, rapidly eroding the fractured ancl loose surface rock down for aboul 2 m. At that point the erosion slowecl down and the lower shoreline was formed. This lowering is likely to have occurred about 1890. During stage IV the rock barrier at the Útfall was gradually lowered by erosion and the lake level clropped at the same rate. In the present century the glacier has been retreating. Since about 1960 the passage between Fögrufjöll and the Skaftárjiikull glacier has captured more and more of the glacier water constantly reducing the inflow into Langisjór. This stage came to an encl in 1966, when both the glacier rivers previously flowing into Langisjór had been captured by the drainage between Fögrufjöll and Skaftárjökull. The present staage, V, began at this point. Al- most immediately the lake level clroppecl about half a meter and it is probable tliat in the future the lake level will remain in the same elevation as the rock barrier in the Útfall or even be lower. 6. ON GLAGIAL AND CLIMATIC VARIATIONS In this chapter we will discuss glacial and climatic variations in the light of tlie investiga- hœð - elevation my s.- mojs. Fig. 8. Changes in the water-level of lake Langi- sjór during the last 100 years. Myncl 8. Vatnsborðsbreytingar í Langasjó siðast- liðin 100 ár. 292 JÖKULL 17. ÁR
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