Jökull - 01.12.1992, Page 27

HOSKULDS VATN:
CAUSE OF UNUSUALLY HIGH LAKE LEVEL IN 1991
Eysteinn Tryggvason,
Nordic Volcanological Institute, University oflceland,
101 Reykjavík, Iceland
abstract
Lake Höskuldsvatn is located about 10 km south-
east of Húsavík, North Iceland, at an elevation of262
m- This lake has no surface outflow, and the lake level
varies and is controlled by precipitation, evaporation
and possibly by variations in the subterraneous path
°fflow from the lake. During the summer of 1991, the
lake level was higher than previously observed and
the cause ofthis anomalous lake level was ofconsid-
erable concern. An investigation ofthe lake behavior
mdicates that the primary cause ofthe high lake level
in 1991 was heavy precipitation during the preceding
years, but ground deformation ofthe recent Krafla vol-
canic episode may have reduced the permeability and
the hydraulic gradient of the ground water drainage
fi-om the lake.
introduction
The lake Höskuldsvatn lies about 10 km southeast
°f Húsavík, north Iceland, in a tectonic depression
controlled by the Húsavík fault, a NW-SE trending
right lateral fault, a branch of the Tjörnes transform
fault zone. Its elevation is 262 metres according to
official maps and it has no surface outlet. The to-
Pography gives the catchment area of the lake about
30 km2 (Figure 1), but because of high permeability
°f the Pleistocene and Holocene volcanic formations
°f the area, rain water seeps into the ground, and the
effective catchment area is partly determined by the
groundwater flow. The lake elevation is controlled by
precipitation of the catchment area, evaporation and
permeability of the geologic environment. This eleva-
tion varies considerably, but it has not been measured
systematically.
It is known, however, that the lake level is higher
in the spring than later during the summer, and the old
road along the south shore of the lake has frequently
been under water in early summer, while the winter
snow was melting, but a bypass road was used under
those circumstances. The lake level did not subside
as usually during the summer of 1991, and was very
high throughout the summer. In late July, the old road
bypass became flooded, and a new track had to be
laid to make the road passable. This had to be repeated
later in the summer of 1991 because of further increase
of the lake elevation. This high stand of the lake has
never been observed since the road was opened almost
60 years ago, indicating some unusual circumstances
in the hydrology of the area. However, on October 28,
the lake level had subsided about 34 cm since August
(Svavar Jónsson, pers. comm.)
There have been some speculations about the cause
of this increased lake elevation, and several possibil-
ities have been suggested. A frequently mentioned
possibility is an increase in the precipitation. Another
is increased melting of perennial snow fields in the
mountains. The lake bottom may have become less
permeable because of sediment influx, and even the
regional permeability may possibly have decreased
because of tectonic processes that have compressed
the subsurface drainage channels.
JÖKULL,No. 42, 1992 25