Jökull - 01.12.1967, Qupperneq 29

The Changing Level of Hagavatn and Glacial Recession
in this Century
GUTTORMU R S I GBJARNARSON,
HATIONAL ENERGY AUTHORITY, REYKJAVIK, ICELAND
ABSTRACT
The present paper deals with the history of
Hagavatn and the “jökulhlaups” from there.
The abandoned shore lines al Hagavatn are
found to be in a good correlalion with certain
water ivays, tuhich have deterrnined the eleva-
t.ion of the lake level at different times. Tlie
glacier retreat has brought about changes in the
drainage conditions at the southern margin of
Langjökull; thus Tungufljót river has captured
luater from the rivers Sog and Brúará. Data
concerning the recession of Langjökull in this
century have been compiled, and the clecrease
in its area and volume has been measured on
maps ancl calculated to be about 14% in area
and about 41 kms in volurne. The increase in
drainage from the glacier caused by the ice re-
cession has also been calculated to be about
14.5% of the discharge of Tungufljót and about
5.5% of the discharge of Ölfusá at Selfoss.
I. THE HISTORY OF HAGAVATN
The Hagavatn basin
The Hagavatn lake is located at the southern
margin oi' the Langjökull glacier between Haga-
fell and the Brekknafjöll Range (Fig. 1). Its
elevation above sea level is 436—7 m and its
present area is 5.0 km2. Its greatest measured
depth is 26 m (Green 1952), but no bathymetric
map is yet available.
To the SW Hagavatn is bounded by Lamba-
hraun, 678 m a.s.l. It is a regularly formecl
shield volcano, which has filled up the valley
depression NW of the Brekknafjöll Range. The
lava from the Lambahraun crater has also flow-
ed to SE through some cols in the Brekknafjöll
Range clown to the uppermost farm on the west
side of Bjarnarfell, where it is named Úthlídar-
hraun. No suggestion will be made as to the
depth of the Hagavatn basin before the forma-
tion of the Lambahraun shield volcano. Neither
can it be suggested how the basin was drained,
because either it has been without surface
drainage or all drainage records have been buri-
ed by the lava flow. The present outlet f'rom
Hagavatn is at the elevation of 436 m and the
col Brúarárskörcl at the southern end of the
former basin lies approximately at the same
elevation. Had either of these passes been the
outlet, the Lambahraun shielcl volcano would
have been forrned in a very big lake filling the
former Hagavatn basin, but the lava formation
does not show any signs of having flowed into
a lake or water. There are some possibilities
that the Hagavatn basin had no surface drain-
age before the formation of Lambahraun as the
bedrock is very leaky in tliese surroundings.
Yet it is more likely that the drainage was
through some col in tlie Brekknafjöll Range
between Mosaskardsfjall and Kálfstindur and
from there flowing in a SW direction to the
Brúará river, as the range is lowest in that part,
and lias been almost totally submerged by the
lava flow.
The Lambahraun shield volcano is post-
glacial. Its age can be estimatecl with the aicl
of tephra layers. In these environs the light
coloured tephra layers H3 and H4 are found in
soil profiles with three black tephra layers in
between. H3 is 2820 ± 70 and H4 is 4030 ± 150
C14 years in age (Thorarinsson 1964). In soil
profiles on the Lambahraun lava the tephra
layer H3 was found and also the uppermost of
the three black tephra layers. By studying the
lava edge I happened to find the sublayers of
it in three ephemeral stream channels. Two of
these places are situated in the inlet of the
JÖKULL 17. ÁR 263