Response of small mid-mountain rivers to human impact with particular reference to the last 200 years; Eastern Sudetes, Central Europe

Abstract

In the mid-mountains of Central Europe relict forms of periglacial morphogenesis occur, including debris slope covers. However, the Holocene forest succession limited sediment transfer towards valley systems. The eastern, mid-mountain part of the Sudetes, characterised by a dense stream network and by the local occurrence of metallic and gold ores, was already settled by humans in early medieval times. Timber from the forested slopes was used to produce charcoal and streams acted as energy sources for smelting plants and smithies. Agriculture developed on less inclined slopes. Temporal clearances on steep slopes and replacement of primeval beech forest by spruce trees, influenced water circulation on slopes. The exposed slope covers were transferred to valley floors during high precipitation events. Consequently, they were selectively redeposited within the floodplain, changing the former channel pattern, especially at confluences. The introduction of cultivation on slopes triggered intense soil erosion so that agricultural terraces were built in order to prevent surface wash. The terraces are often strengthened with stone walls up to 4.5 m high. Terraces acted as ‘sediment traps’, storing the washed material within the slope. The layers of fine-grained colluvial sediments, which can be linked with agriculture, cover the older, debris slope covers. The colluvium is especially thick at the terrace edges. Despite the protective role of terracing, part of the washed material was transferred to valleys, often becoming overbank deposits, covering older gravel and boulder alluvium. The decline of iron ore exploitation and processing, at the beginning of the 19th century, together with agricultural withdrawal, which was especially radical after the 2nd World War, resulted in successive renaturalisation of the environment being followed by slope stabilisation and deepening of bigger river channels. The results led to the conclusion that a strong human impact on the slope–valley system in the mid-mountain part of the Eastern Sudetes influenced changes in the dynamics and morphology of small rivers to a much higher extent than the climate variations, especially during the Little Ice Age.