Jökull - 01.01.2009, Page 69

Deglacial and Holocene sediment distribution in Hestvatn, South Iceland
Figure 1. Landsat images of the research area. a. River Hvítá drains Hvítárvatn by Langjökull, and flows east
of Hestvatn. Simplified configuration of the Búdi moraines is drawn. Two of the outlet glaciers of Langjökull,
E and W Hagafellsjökull are indicated. b. River Slauka is the only outflow of Hestvatn, but when water level of
Hvítá is high, water is diverted into Hestvatn via Slauka. Location of ice-proximal delta on the southwest shore
of Hestvatn is shown. – Gervitunglamyndir af rannsóknarsvæðinu. Hvítá rennur úr Hvítárvatni við Langjökul
og austan og suður meðHestvatni. a. Einfölduðmynd af Búðaröðinni. b. Slauka er eina útfall Hestvatns. Þegar
vatnsstaða Hvítár er há flæðir inn í vatnið um Slauku. Suðvestur af Hestvatni er þykkir bunkar af sjávarseti
sem líklega hafa hlaðist upp framan við jökul.
ure 2). However, the glacial river Hvítá flows oc-
casionally into the lake via Slauka, either when ice-
dams form on the river or increased glacial meltwa-
ter, caused by spring floods, or surges of East or
West Hagafellsjökull (Figure 1a), outlet glaciers of
the ice cap Langjökull (e.g. Sigbjarnarson, 1976). In-
flow from Hvítá is known to take place on an ap-
proximately bi-decadal timescale (Harðardóttir et al.,
2001a).
METHODS
A seismic survey was conducted on Hestvatn in 1994
(Harðardóttir et al., 2001a). Over 100 km of seismic
reflection data were collected from the survey boat
Bláskel, equipped with a Geopulse Boomer system.
The system operated at 175 J and the data were fil-
tered for frequencies between 0.75 kHz and 20 kHz.
The boat was positioned with a Trimble 4000 DS Dif-
ferential GPS system with a ±20 m resolution. Sedi-
ment thickness, calculated using freshwater sound ve-
locity of 1465 m s!1, represents a minimum thick-
ness. Seismic lines were spaced approximately 100
m apart along SSW-NNE lines, but more sparsely dis-
tributed E-W lines were surveyed (Figure 2a). Black
and white lines in Figure 2a indicate the track of the
boat and surveyed profiles, however, in a few places
the equipment gave unsuccessful results. Harðardóttir
et al. (2001a) analyzed the seismic profiles but here
we extend the survey and add ArcGIS procedures to
more thoroughly analyze the data.
The seismic profiles were digitized and the seis-
mic data processed using Geographic Information
System (GIS) for further analysis. Raster and vector
datasets were assembled with a consistent projection
(Iceland Lambert) and datum (ISNET93/WGS84).
Datasets included point shapefiles for bathymetry and
thicknesses of sedimentary units, raster versions of
1:50,000 maps of the Hestvatn area (sheets 1712-IV
and 1713-III) from the National Land Survey of Ice-
land, the Hestvatn shoreline, shapefiles for core loca-
tions and seismic transects, Landsat 7 satellite images
of the research area.
JÖKULL No. 59 69