Über Alter und Bildung von Talmäandern

Abstract

An analysis of research work, executed over the last two decades, on the problem of meanders shows the great advance made towards an understanding of the formation of free meanders (;i. e. meanders formed by a river approaching base level). Particularly important in that respect was experimental work, notably that carried out by the U. S. Waterways Experimental Station in Vicksburg. This analysis shows on the other hand that the explanation of entrenched meanders (meander valleys) is still too much under the influence of the cycle theory of W. M. Davis according to which entrenched meanders are derived as „inherited meanders from free meanders on plateaux. In addition research workers were much concerned to find single factors that were responsible for meander formation and some of these factors were even used for a classification of meander valleys into groups such as Gesteinsmäander (lithologically conditioned meanders) or Kluftmäander (meanders conditioned by structural weaknesses). In contrast investigations of the geomorphological development of the valley sections where meanders occur were neglected. The meander valleys which were formed during the Pleistocene period can only be understood when studied in conjunction with the complete climatic and tectonic evolution and Pleistocene landscape development as a whole. Among the items to be considered are: climatic changes, relationship between precipitation, run-off and evaporation, soil formation, fluviatile deposition and erosion, and eustatic changes of the sea level. For an illustration of this point the body of the author's own research on meanders of Alpine rivers (Inn, Alz, Lech, Ticino, Rhine) formed after the climax of the Würm glaciation during the stage of dissection of the sheets of outwash material, is drawn upon, as well as the work of Elisabeth Kremer who investigated anew the classic meanders of the middle Mosel (Moselle) valley. In the Inn valley (Fig. 1) it is shown how, after the formation of the fluvio-glacial gravel sheet, deposited at the height of the Würm glaciation, during the retreat of the ice, dissection took place in a strictly regular sequence whilst the relationship between down-cutting and side erosion was reversed. The first stage was the formation of Trompetentäler (trumpetshaped valleys) whose banks decrease in height downstream and whose flood plains widen and finally grade into more recent gravel fans; as the second stage followed the formation of Gleitmäander (slip-off meanders) which, by pronounced down-cutting power, occurred in the upper parts of the gravel sheets of still considerable gradients. The valley section where meanders were found gradually migrated downstream and in each case gradated into a trumpet-shaiped valley. The meanders of the Inn are consequently not inherited free meanders, but valley meanders which were formed, while considerable downward cutting and side erosion took place, as a result of adjustment after deposition of the fluvio-glacial gravel sheets with their steep longitudinal profiles. They thus illustrate a climatic, not a tectonic, cycle. This type of fluvio-glacial meander formation during the Late-glacial period can be shown to have taken place also on the gravel sheets of the Alz, Lech, and Ticino rivers (Fig. 2). The meanders of the Rhine at Mannheim and Worms (Fig. 3) are also such valley meanders - though their incision is less pronounced - which were formed when the river changed from depositing the niederterrasse (low terrace) to the Late-Pleistocene period of erosion. The meanders of the Mosel belong to the type of meander valleys of the Hercynian massifs of Middle and Western Europe, formed in the course of a longer Pleistocene valley development (Map 6). Of the 6 gravel terraces of the Mosel, 5 are derived from the climatically conditioned depositions in the periglaeial climatic zones of glacial periods (Günz, Mindel, Riss I, Riss II, Würm), only one is due to tectonic movements. The wide gravel terrace of the Mosel deposited during the Mindel glacial period shows that meander formation had not yet begun. It commenced with the climatic change after the height of the Mindel glaciation due to combined downward and side erosion, the force of which was increased by the tectonic uplift during the Mindel-Riss interglacial period. The following three glacial periods interrupted the normal development of these erosion meanders by depositions. This change over from periglacial deposition to Lateglacial erosion is of decisive importance also for other meander valleys of Middle and Western Europe.