Abstract
The impact of transient climate change, for example at glacial-interglacial transitions, on the alluvial valley of the lower reaches of larger river systems has become a classic topic of fluvial geomorphology and quaternary geological study. The process of contraction of Holocene river activity into a narrower channel belt than in
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LGM counterparts of the last cycle, links to terrace flight formation over multiple glacial cycles in inland reaches. Not only were valley reaches affected by the process: also the lower reaches traversing shelf areas were, at a critical time just before their transgression. It is important to get fluvial style change mapped and dated accurately, to contribute to Source-2-Sink style sediment budgeting studies and be able to quantify sediment routing over longer time scales ‘from the record’ rather than ‘from assumptions pasted in models’.
In the Netherlands, in our current time, we are favoured by a very well resolved Late Pleistocene and Holocene geological record. This is due to favourable palaeogeographical circumstances and long research tradition from academia and applied fields. The sheer amount area mapped in detail, and sheer number of ‘to-be-integrated’ studies made it fruitful to set up and fill a GIS system
for the reconstruction of valley evolution since the LGM (Cohen et al., 2012), expanding earlier such work for the Holocene Rhine delta (Berendsen et al., 2007). The reconstruction now covers the Valley evolution of the Rhine since the LGM, continuous in space and time from upstream reaches in the German Lower Rhine Embayment (Erkens et al., 2011), through the Dutch-German border area (e.g. Kasse et al., 2005; Janssens et al. 2012), the central delta (Busschers et al., 2007; Gouw & Erkens, 2007) to the present river mouth and beyond (Hijma et al. 2009; 2012). To produce the map, we filled a digital catalogue documenting each mapped element (paleomeanders, terrace-fragments) in the valley.
Reconstruction of reworking as part of the valley evolution is an intrinsic part of the encoding in the GIS.
The main finding in the Lower Rhine is that the studied transition appears to last longer and to be slower than in smaller rivers of NW Europe. From the LGM to after the transgression, the area was subsiding relatively fast owing to the near-field GIA circumstances on top of background tectonics. All the time the area was fed river water and sediment by the Rhine: a sizable mixed-load river system. During the period of interest, climatic amelioration made vegetation go through a distinct succession. Not only does this provide extra dating control beyond direct methods of 14C and OSL, but also does this provide opportunities to reconstruct local and regional hydrological changes and relate this to the changes in fluvial style and reworking (scour, incision, levee deposition, lateral migration, floodplain soil formation). Extensive areal palaeogeographical reconstruction (GIS map and catalogue) shows the Lower Rhine's valley evolution from LGM to Holocene in unprecedented detail
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