Bestimmungsversuch des Ria-Begriffes durch das Kriterium der Fluvialität

Abstract

The problem of the scientific term 'ria' was, even in the beginning, primarily of a definitional nature. F. von Richthofen basically cleared up the problem of origin, defining rias as landforms which were formed by the erosion of running water and filled by rising sea level. The ria problem as such can be summarised in the question: Which bays are rias and which are not? - Using the example of French marine bays and several important earlier definitions, the answer is here given that rias are valleys and valley systems excavated by fluvial action and partially flooded through glacial isostasy. 'Fluvial excavation' means in this context all linear terrestrial (i. e. not marine) flow processes, insofar as they form valleys with similar types of slope to supra-terranean rivers (in contrast to subterranean karst streams and melt water channels under the ice), for example certain solifluction processes or torrential flow are included. 'Flooding through glacial isostasy' means that rias were formed primarily by the worldwide rise of sea level, bound up with the melting of inland ice masses, but not through tectonic land subsidence (this is always of secondary, if any, importance for ria flooding). Only those bays which were subjected to fluvial landform processes at least as late as the last cold period should be called rias: because of this, rias cannot be significantly deeper than 150 metres. 'Partial flooding' is aimed at the degree of flooding as the expression of a particular relationship between the original fluvial form and the contemporary marine form which, in the case of rias, should be so developed that the fluvial valley is still reflected in the structure of the bay. Mono fluvial rias are formed by the partial flooding of linear valleys in such a way that the valley partings with in the bay are hardly shortened. Poly fluvial rias are formed by the partial floodings of valley bundles in such a way that the valley partings within the bay are sharply reduced and often broken up into islands. Pan fluvial bays are formed by the partial flooding if valley landscapes (Talschaften) where the internal valley partings are largely eliminated (transgressively or abrasively). Despite being genetically related, these bays cannot be regarded as rias, because their fluviality has been physiognomically lost. Estuaries, here understood as the accumulation mudflat on tidally-influenced river confluences, are to be regarded as a bay habitus, which can often occur in rias but not exclusively. Rias can show other habitus forms: Nehrung formation, transition to freshwater forms, or full fossilisation.