Paper in prepare: Complex transformation of the geomorphic regime of channels in the forefield of the Moravskoslezské Beskydy Mts: case study of the Morávka River (Czech Republic)

This paper presents a complex analysis of both the contemporary and historic development of the geomorphic regime of the transformed reach of the Morávka River in the Czech Carpathians. The assessment concentrates on the conditions and causes of the channel development of the last c. 200 years, as compared to the state of European channels, especially those of the Carpathian zone. The Morávka River pattern has undergone a rapid change in the last 50 years, namely in connection with the active channel narrowing and massive deep erosion. The original anabranching river pattern has gradually been substituted by a simple narrowed channel that incised as deep as the bedrock (see Fig. 1).

 Fig. 1: The development of the Morávka river pattern between 1.6 and 3.5 r. km in the period of 1836 – 2010; A – state on the map of the Second Military Mapping from the period of 1836-52; B – state on the map of the Third Military Mapping from 1876-78; C – state captured on the aerial image from the year 1937; D – state on the aerial image from the year 1955; E – state on the orthophoto from the year 2010.

At some parts, the original river bottom has lowered as many as 8 m in the last 40 years, which points to the incision rate of 12-24 cm per year (see Fig. 2).

Fig. 2: Cross profile at 2.31 r. km in the area of the canyon of the Morávka River with the identification of deep erosion starting in the year 1992.

These changes have been caused by a strong anthropogenic impact in the form of the river channel controlling, bank stabilising, and level, weir and valley dam constructing. Other reasons are related to land cover and land use changes. An important role in the Moravskoslezské Beskydy Mts is played by the period of the so-called Walachian colonisation of the second half of the 15th century and the 16th century characterised by deforestation, land cover degradation, accelerated runoff and gully erosion initiation. This period was replaced by the phase of reforestation in the first half of the 20th century when the stabilisation potential sedimentary material for Beskydian channels initiated along with deep river bed erosion. A great influence on the contemporary processes is also exerted by the geological predisposition of the Carpathian flysch lithology in channel bedrock. It is particularly the occurrence of claystone layers that are little resistant to water erosion. At the present time, the deficit of transportable sedimentary material along with increased transport capacity of the incised riverbed make conditions for constantly intensifying erosion processes in the Morávka channel. 

Fig. 3: A – Comparison of average incision and discharge values of selected European rivers; B – Variability in average incision rates of Carpathian rivers and remaining European rivers (including Alpine rivers); C – Variability in the discharge of Carpathian rivers and remaining European rivers (including Alpine rivers); p – result of non-parametric statistical Mann-Whitney U-test presenting the significance and insignificance of differences, the box plot shows the span of measured widths between the lower (25%) and upper quartiles. (Created based on: Kondolf et al., 2002; Lach and Wyżga, 2002; Marston et al., 1995; Martín-Vide et al., 2010; Preciso et al., 2011; Rinaldi and Simon, 1998; Rinaldi, 2003; Rovira et al., 2005; Surian and Cisotto, 2007; Surian and Rinaldi, 2003; Surian et al., 2009; Uribelarrea et al., 2003; Wyżga, 1993; Wyżga, 2001; Zawiejska and Wyżga, 2009).

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