Euro-limpacs Deliverables

ABSTRACT - DELIVERABLE 98

Report - Effect of braiding and sedimentation on mountain rivers

Abstract - Deliverable 98

Report - Effect of braiding and sedimentation on mountain rivers
- part 1: braiding -
- part 2: sedimentation -

PART 1 BRAIDING

  1. Effects of river restoration on hydromorphology, floodplain vegetation, riparian ground beetles and benthic invertebrates were studied in medium-sized mountain rivers in Cen-tral Europe.
  2. Seven restored sections were compared to seven nearby non-restored sections. In all study sites restoration generated multiple-channel patterns, while channels in the non-restored sections are straight, single-channel sections.
  3. Channel elements (floodplain mesohabitats) and aquatic microhabitats were recorded along 20 transects per section. All habitat types available for floodplain vegetation, ripar-ian ground beetles and benthic invertebrates were sampled individually (several samples per habitat type); overall species lists per section were generated by averaging all sam-ples taken in the same habitat and section, multiplying it with the proportion of the habi-tat at the section and summing up the resulting lists. Two hydromorphological indices and 13 biotic indices were calculated for each section.
  4. The number of floodplain mesohabitats was significantly different between single- and multiple-channel sections, while there was no significant difference in the numbers of aquatic microhabitats.
  5. Floodplain vegetation reacted most strongly to restoration (four out of five indices were significantly different between single- and multiple-channel sections), followed by riparian ground beetles (one out of three indices). The indices calculated for the benthic inverte-brate fauna were not different between single- and multiple-channel sections.
  6. Cluster analysis with quantitative taxa lists of the floodplain vegetation resulted in a clear separation of single-channel sections and multiple-channel sections, indicating that channel form is the main determinant of community composition. For benthic inverte-brates, single- and multiple-channel sections of the individual rivers were always clus-tered together (channel form does not determine community composition). The analysis of the ground beetle taxa lists showed intermediate results.
  7. The strong reaction of floodplain vegetation to restoration is mainly due to several addi-tional habitats generated, while riparian ground beetles react mainly to the generation of gravel bars. The failure of the benthic invertebrate fauna to react to restoration is partly due to comparatively small changes in aquatic microhabitat composition.
  8. Other reasons explaining the differences between organism groups include differences in dispersal abilities and source populations; while for floodplain vegetation and riparian ground beetles mainly the habitat matrix is determining community composition, benthic invertebrates are also depending on the medium water
  9. Early and local effects of hydromorphological restoration are best detectable by riparian vegetation, while long-term effects and those on larger scales might be better reflected by benthic invertebrates.
  10. PART 2 SEDIMENTATION

    One of the topics in Euro-limpacs is the effect of land use on river ecosystems. Contrary to other findings which link erosion rather with deforestation and changes from woodland to rural areas, afforestation with spruces increased erosional effects in the investigated catchment. Due to the sensitivity of these trees to soil-moistness the forests are artificially drained, thus creating a steady source of fine sediments (sandy deposits and fine to medium-sized gravel) which are transported into the rivers. Killingseder (1998) demonstrated that the huge amount of mobile sandy substrates in the river Waldaist is mostly caused by surface runoff and drainage measures in coniferous forests. Siltation can be defined as the deposition of fine sediments either on the surface of the stream bed or within a gravel substrate. Under natural conditions the balance of erosion and sedimentation creates a diversity of sediment patches over the river bed, which is vital for the maintenance of high habitat diversity and for different life stages of some species. Problems arise through high deposition rates, smothering of coarser patches with finer sediments, and ingress of finer materials that may deplete oxygen levels either through a reduction in throughflow rates or, in the case of organic particulates, by their own use of oxygen (e.g. Buddensiek, 1991, 1992; Buddensiek & Ratzbor, 1995; Richards & Bacon, 1994).
    Reviewing the literature it is obvious that siltation is a major topic in riverine ecology. It is widely recognized that anthropogenic activities such as agriculture (e.g. Walling 1990; Richards et al., 1996), forestry (e.g. Scrivener & Brownlee, 1989; Murphy & Milner, 1997), groundwater abstraction (e.g. Bickerton et al., 1993) and periodic in-channel management activities (e.g. Brookes, 1986; Hearne & Armitage, 1993; Doeg & Koehn, 1994) can result in significant changes to in-channel deposition, storage and erosion of fine sediments. The ecological consequences of siltation comprehends the following topics:
    • substrate composition
    • substrate stability
    • quantity, quality and availability of food resources
    • clogging of hyporheos
    Both suspended solids and condition of the substrate, particularly with regard to siltation, are important for many species. For example, high suspended solid concentrations can affect the feeding and health of individual species either indirectly through increased turbidity of the water, or directly through clogging of gills. Siltation is a potential problem both with regard to access to suitable substrate – for example, for the establishment of the freshwater pearl mussel (M. margaritifera) – and with regard to egg and fry survival in populations of salmonid fishes. Additionally we propose a decrease of grazers due to the lack of coarse stable substrates.

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