The soils of the river valleys with an active erosive downcutting (on the example of the Prut river basin, Precarpathians)
Disaster floods on the Carpathian rivers and surrounding areas as a result of radial and lateral erosion have predetermined to significant changes in geomorphological levels: the rivers bottom in some areas deepened to 100 cm or more, the coastline has shifted to 15–20 m, landslide processes first of all on the shores was invigorated. Due to the flooding in 2008 the soil and vegetation cover of the floodplains was completely destroyed, where the surface alluvial sediments and, often, the bedrock of the river valley went out. To prevent the river influence on the floodplain, the floodplain has become a terrace, so from this moment the alluvial soils have changed their place in the classification system. Pedogenesis in the river valley is certainly correlates with the specified geological and geomorphological processes, and therefore one question arises: in which taxonomic of soil classification the soil is, which now is located outside of floodplain, but the time that has passed from this moment is too short for the genesis of the first terrace soil (formed before our eyes). Shall we talk about the ancient age of alluvial deposits if they were formed in the last 100–1000 years? It is apparently that we need to consider the simultaneous genesis of thesediments, the landforms and the soils under the influence of the geological processes. We have studied the valley of the river Lyuchka, belonging to Prut River basin. We have described the soils that are placed on the ecotopes that have a direct contact with a coastline of the river, on different height levels above the top of watercourse, as well as the cuts of zonal soils which were not under the influence of the river during of the historical time. Soil samples for analytical studies have also been selected. We have studied the cuts of such soils: 1) В-6, К-1 and К-2 – zonal; 2) В-1, В-2, В-5 and В-7 – soils placed on the first terrace; 3) В-3, В-4, В-2а and В-3а – soils placed on the floodplains. The results obtained are summarized as follows. Soils of Lyuchka valley are developing on the alluvial deposits under which the bedrock are placed that in the area of research is clayey flysch; all these soils are on the development stage. Soils of the first terraces are Fluvisols on the complex alluvial deposits, mostly they have the average thickness and the soils are characterized of such morphometric features: humus horizon is from 33 to 66 % of the thickness of profile and transitional horizon is from 34 to 67 % of the thickness of profile. The problem of their selection is establishing of the age, because for alluvium is not specified definitions of "old, ancient". It really could be aged of alluvial deposits are a hundred-thousand years or even as whole Holocene. Another problem is a quick output time of the floodplain from the influence of the river and the formation of the first terrace. This requires a change of the name of the soil (maybe the output from Fluvisols). It remains an open question whether this change of belonging of classification reflects the inner essence, and whether all the soils of river valleys should be admitted young, genetically related (monogenesis) and therefore whether they should occupy one position in the classification system.
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