Jökull - 01.01.2005, Side 75

Reviewed research article
A late Holocene jökulhlaup, Markarfljót, Iceland:
nature and impacts
Kate T. Smith1,2 and Hreinn Haraldsson3
1Institute of Geography, School of Geosciences, University of Edinburgh
2Institute of Earth Sciences, University of Iceland
3Public Roads Administration, Reykjavík, Iceland
emails: kate@raunvis.hi.is and hrh@vegag.is
Abstract — Approximately 1200 years ago (c. 1230 14C yr. BP) a mature birch wood in southern Iceland
was devastated by inundation in a catastrophic jökulhlaup flowing westwards from Katla. This flood was the
youngest prehistoric flood in a series of Holocene jökulhlaups that covered the Markarfljót-Landeyjar area of
southern Iceland. Sedimentological analysis presented here indicates the flood was associated with an eruption
of the volcano Katla, similar to historical jökulhlaups along the south coast of Iceland. A new radiocarbon
date, tephrochronological studies and observation of the remaining tree stumps combined with interpretation
of previously published data shows that the trees were around 60–100 years old when inundated and that much
of the lowlands of Landeyjar were probably similarly vegetated at this time. Deposits and scoured bedrock in
similar stratigraphic locations upstream may relate to passage of the flow along the main Markarfljót valley
from northwest Mýrdalsjökull. From a hazard perspective it is critical to establish that such medium-scale
events have occurred in the past and their nature and impact, particularly since they are generally censored
from older Holocene sedimentary records by later geomorphic activity.
INTRODUCTION AND CONTEXT
Jökulhlaup-dominated sandur plains dominate the
south coast of Iceland (Maizels and Russell 1992).
The impact of these jökulhlaups on areas that have
been subjected to repeated historical flooding (e.g.
Mýrdalssandur, Skeiðárarsandur in southeast Iceland;
Larsen 2000, Russell and Marren 1999) is well under-
stood and hazard plans for such future events in these
areas are well developed. Recent activity in the Katla
Volcanic System has increased awareness of potential
hazards associated with future Katla eruptions (Guð-
mundsson et al. 2005).
Throughout the Holocene, interaction between the
volcano Katla and the overlying ice cap of Mýrdals-
jökull has triggered many jökulhlaups, to the south
and east during historical and late prehistoric time and
to the west prior to settlement (Figure 1). Historical
records of flood activity to the south and east of the ice
cap document varying scales of flood from large-scale
events (such as the 1918 flood which reached 300,000
m3/sec, e.g. Tómasson 1996) to very much smaller
floods (such as the Sólheimajökull flood of 1999,
Einarsson 2000). No known historical Katla floods
have occurred westwards along the major braided
river Markarfljót. However, sedimentary records of
flood activity in the Markarfljót valley indicate that a
series of large, valley-filling floods occurred in pre-
historic times (Smith 2004, Larsen et al. 2005). This
record only includes the largest floods since subse-
quent jökulhlaups and fluvial activity have removed
and buried traces of smaller events which did not wash
deposits onto the less eroded valley sides. This limita-
tion of the sedimentological chronology is important
when considering flood frequency and scale in haz-
ard assessment. This paper presents key evidence of
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