Jökull - 01.01.2004, Page 54

Jessica Black et al.
Figure 13. Relationship between suspended sediment load and discharge measured at the outlet of Hvítárvatn
each summer from 1965 to 1999 (Pálsson and Vigfússon, 1996). There is no statistical correlation between dis-
charge and suspended sediment load, presumably because the suspended sediment in the lake is derived from
glacial erosion process, not fluvial erosion. – Samband svifaurs og sumarvatnsrennslis við útstreymi Hvítár-
vatns frá 1965 til 1999 (Pálsson and Vigfússon, 1996). Ekki er um að ræða tölfræðilega fylgni á milli rennslis
og svifaurs, sennilega vegna þess að svifaurinn í vatninu er tilkominn vegna jökulrofs fremur en árrofs.
estimates the suspended load loss because glacial ero-
sion products entrained during the Little Ice Age are
still being delivered to the lake. Consequently, we use
it to derive a maximum erosion rate.
The minimum mass of sediment delivered to
Hvítárvatn after the receding Icelandic Ice Sheet left
the basin is 35 x 1013 g, the mass directly measured
by the seismic survey. The area outside our survey is
estimated to contain an additional 35 x 1013 g of sedi-
ment. Combined with the estimated loss of suspended
sediment, the maximum estimate for the total mass
of sediment delivered to the lake since deglaciation is
121 x 1013 g (Table 2). The Hvítárvatn catchment is
dominantly comprised of basalt (ca. 2.8 g cm−3), with
lesser amounts of hyaloclastite (ca. 2.6 g cm−3) and
rhyolite (2.3 g cm−3; densities from Hall, 1996) in
the mountains to the east; weathered rock is even less
dense. We use a conservative average bedrock density
of 2.7 g cm−3 to calculate catchment-wide erosion
rates. If the erosion products were derived from soils,
the surface lowering rates would be much higher. Ice-
landic soils have bulk densities that average ca. 0.6 g
cm−3 (Óskarsson et al., 2004).
Assuming the entire catchment contributed
equally to the sediment flux to Hvítárvatn yields
an average Holocene surface bedrock lowering rate
across the catchment of between 2 and 5 cm ka−1 (Ta-
ble 2). Most sediment pathways to Hvítárvatn origi-
nate as meltwater streams from Langjökull, whereas
streams entering the lake that drain unglaciated ter-
rain have almost no suspended load. Consequently,
most of the sediment delivered to Hvítárvatn is proba-
54 JÖKULL No. 54