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| Titel |
Pitfalls in estimating past sedimentation rate in floodplains of aggrading rivers |
| VerfasserIn |
Tomas Grygar, Tereza Novakova, Ivo Svetlik, Martin Mihaljevic, Zdenek Macka |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250033067
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| Zusammenfassung |
| In some recent studies of flood plains of European aggrading rivers, facial and architectural
dependences of apparent sedimentation rates in have been underestimated. The facial
dependence is either a consequence of slower and finer sedimentation in a distal floodplain or
coarser and faster sedimentation in the closer vicinity of a river channel. This basic
knowledge is not considered if sediment samples are obtained from drill cores and cannot be
evaluated by a sedimentologist, or a large number of 14C data is statistically processed while
the depositional environment of each individual dated material is not adequately
considered.
We have studied sediments from about 1/3 of the total width of the floodplain of the
lower course of the Morava River near Straznice in the south-eastern part of the
Czech Republic (upstream watershed 9146 km2). We collected samples from up to
5 m thick floodplain sediments in the river erosional banks and up to 4 m long
drill cores in the floodplain, and subjected them to geochemical and granulometric
analyses. Their absolute dating was performed by 14C analysis of wood debris or
charcoal (Middle and Upper Holocene) and the sediment from the last century was
identified due to its regional industrial contamination by Pb and Zn and magnetic
particles. The apparent aggradation rate was calculated to range from 0.2 to 0.6 cm/y
and we spot that it has been indirectly proportional to the sediment clay content.
As the younger sediments were coarser than older ones in the studied part of the
floodplain area, our results "pretended" an increase of the sedimentation rate during the
last millennium. This bias became obvious after evaluation of the change in the
Morava River channel system in the last approximately five centuries according to the
sediment record, of which the final stage has been depicted in historical maps from the
last two centuries (reliable older maps are not available). Instead of accelerated
aggradation, we have in fact revealed a gradual change of the Morava River from a
complex system of several, probably meandering branches during the last several
centuries to the actual single channel passing through the area we have studied
recently.
Another systematic change of floodplain sediments, which we revealed in the studied area
of Morava River, seems to have proceeded during the last 4 millennia: coarse, mostly sandy
sediments found on the base of studied profiles are usually older than 4 ky BP; poorly sorted
sandy-silty-clayey sediment is dated to 4 to 2 ky BP; and up to 4 m of better sorted top
sediments have been deposited in the last about 1500 y. Because these sediments point to a
different structure of the former Morava River, we cannot unequivocally conclude that the
loam deposition has started just during the last 1 to 3 ky, as it is almost conventionally stated
for many similar European floodplains on the base of similar results. It is hard to
exclude two alternative explanations: 1) removal of the older loamy sediments by
plain lateral movement of meandering river channels and 2) different floodplain
architecture of our studied part of the floodplain in comparison with the entire past
floodplain. Both these alternative explanations seem sound for the studied Morava River
floodplain.
Therefore we show that the evaluation of sedimentation rate in the floodplain of an
aggrading river must be done with a great care to avoid erroneous description of
the past erosion in the watershed and the subsequent deposition in floodplains. |
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