Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Váně

Diversion of the Ohre River near Postoloprty and geomorphological development of the Chomutovka River Valley

Journal of the Czech Geological Society, volume 38 (1993), issue 1-2, 101 - 109

The graben along the Krušné hory Mts in NW Bohemia with numerous deposits of brown coal and other minerals is known for its complex geological structure. The origin of some structural phenomena is related even to the latest period, i.e. the Pliocene and Pleistocene. The basin is fringed on its NW side by a master fault along which the Krušné hory Mts were uplifted. The latest major movements occurred in the Pliocene (Králík 1989), the displacement being about 500 in high. The origin of drainage patterns, river valleys and accummulations of Quaternary sediments is related to the rise of the Krušné hory Mts. The Chomutovka river incised a canyon about 130 to 200 in deep in the hard rocks of the Krušné hory crystalline complex at the beginning of the Pliocene. Its erosion and transportation strength is believed to have been considerably stronger in some periods of the Pliocene and Quaternary, than it is nowadays. When reaching the basin filled with Miocene sediments, the geomorphology suddenly changes completely. The Chomutovka river spreads over a broad piedmont alluvial plain within the city limits of Chomutov where it forms a pronounced alluvial fan of several levels. Then, in a SE direction, the river cuts into a broad plain built up by Miocene sediments of the central part of the graben. These sediments are covered by several terraces of the Ohře river, composed of sandy gravel. The valley cuts transversely individual terraces prompting the question as to when this valley originated and how it developed.
According to Tyráček (1987), the middle course of the Ohře river represents a classical area that due to abnormal number of terraces seems to have no analogy in continental conditions. I believe that the large terrace system originated due to climatic rather than structural conditions. The major problem in deciphering the development of the Ohře river is to explain its diversion near Postoloprty, This river, already in the Mindel, used to flow through a different valley and followed a different direction, from Postoloprty towards the north into the drainage basin of the present Bílina river. As late as the end of the Mindel, the river turned its course toward the east, into its present valley running through the town of Louny toward Terezín. This unique phenomenon was first recognized and described by R. Engelmann (1922). The major factors influencing the river diversion are thought to be as follows:
i - headward erosion of streams in a basin near Louny
ii - exposure of basalt monadnocks which considerably slowed down vertical erosion and restricted the streamflow to the north
iii - the thick gravel fill of the original valley before a narrowed throat near Počerady
iv - possible gentle uplift of the paleovalley north of Počerady within the horst of the České středohoří Mts.