Anthropogenic impact and morphology channel response of Beskydian gravel-bed rivers: a case study of the Ostravice River, Czechia

Beskydian streams have been recently subject to geomorphological transitions as a result of human interventions in local landscape and stream channels. The main aim of paper Anthropogenic impact and morphology channel response of Beskydian gravel-bed rivers: a case study of the Ostravice River, Czechia is to assess the morphological changes in the studied section of the Ostravice River. In particular, the study concerns four basic areas: (i) river pattern change, (ii) active channel change in the past 200 years, (iii) an analysis of channel incision, and (iv) a comparison of the Ostravice River transformation trends with selected European gravel-bed rivers. A large set of interventions into the fluvial regime of the Ostravice River were man-made projects, specifically the construction of weirs, valley dams and channel control works as bank stabilization affected the fluvial processes operating in the Ostravice River channel. Systematic regulation of the examined section started at the beginning of the 20th century. These interventions influenced the Ostravice River channel which underwent a rapid change in connection with active channel narrowing and incision. The active channel width was measured using the maps of the Second Military Mapping from 1836 to 1852 (on a scale of 1:28,800), the Third Military Mapping from 1876 to 1878 (on a scale of 1:25,000) and aerial images from 1937 to 2010.

This resulted in the original anabranching river pattern has been gradually replaced by a single, narrowed channel. The reach in r. km 0.0–9.0 displays a trend of channel narrowing between 1966 and the present. The previous period was characterised by a stagnation of the channel width. From the point of view of active channel development, a Mann-Whitney U-test at the significance level of α = 0.05 was used to assess the development of the change. The test shows values of no significant difference from the first half of the 19th century until 1966. From 1966 to the present, the Mann-Whitney U-test confirmed a significant difference in the channel width in this area. At some sections, the original riverbed was lowered by as much as 2.5 m in the second half of the 20th century. At other sections, the original riverbed was lowered by as much as 2.3 m in the first half of the 20th century and 2.0 m in its second half. The river reach 9.0–25.0 km displayed a significant change during the period of 1836–1852 to 1955. Between 1955 and 1966, there was no observed, significant change in the channel width. Between 1966 and the present, there has been a notable progressive channel narrowing. The studied reach of the 25.0–38.0 km displayed a significant change in the channel width between 1836–1852 and 1876–1878 and between 1937 and the present. But the first period from the 1836–1852 is characterised by lower values of the Mann-Whitney U-test. No significant change in the channel width is observed during the period from 1876–1878 to 1937. In the river reach of 28.4 r. km, it was observed that the original riverbed had been lowered by as much as 3.2 m in the second half of the 20th century. The upper reach in 38.0–45.7 is characterised by a progressive channel narrowing in the period between the early 19th century and the mid-19th century and between the second half of the 20th century and the present.

The active channel changes and incision of the Ostravice River channel is related to some anthropogenic activities, such as the river control works, damming etc. which blocked the sediment transport in the fluvial continuum. Currently, a deficit of transportable sedimentary material along with a changed morphology of channels with concentrated water flows into the narrow channel intensifies erosion processes in the Ostravice River channel.

When comparing the flysch Carpathian rivers with Alpine rivers, the state of channel changes is probably caused by the unfavourable geological predisposition of the Carpathian Flysch prone to erosion processes.

Contemporary Ostravice River channel: A – in river reach 4.5 r. km; B – in river reach 21.5 r. km; C – in river reach 27.9 r. km; D – in river reach 41.8 r. km.

ŠKARPICH, V., KAŠPÁREK, Z., GALIA, T., HRADECKÝ, J. (2016): Anthropogenic impact and morphology channel response of Beskydian gravel-bed rivers: a case study of the Ostravice River, Czechia. Geografie 121, pp. 99-120. (in Czech with English summary)