THEME 1: RIVER & SEDIMENT MANAGEMENT
Title: Influence of morphological changes on ecology: a cascade of scales
by Prof. Ing. Silke Wieprecht
For the understanding of morphological processes it is not sufficient to only investigate river sections. On the macro scale the catchment plays and important role especially for the available quantity of material fluxes and its composition in the river. Intense river works in the last decades and centuries (straightening, reservoirs, dams, dikes etc.) strongly altered the character of our rivers. Especially the lack of bed load material severely reduced dynamic processes in rivers and ecological important instream structures disappeared.
In some examples the attempt was made to enhance the ecological situation in a river by individual river restoration measures. Depending on the availability of material very different results can be achieved. These effects can also be proven very well with the help of long-term morphological models.
Another option is to bring back sediments in the transport cycle, e.g. by flushing reservoirs. On the one hand this can help to increase morphological dynamics but on the other hand the grain sorting effect upstream of reservoirs changes the characteristics of the river bed also downstream. Due to changes of the material composition the granular structure on the micro and pico scale of the river bed may be heavily altered. It occurs that, although the meso scale structure seems to be good, still not enough acceptable habitats for aquatic species are available.
However, also alterations on the micro and pico scale, like colmation and clogging processes, e.g. due to flushing of reservoirs or changes in wash load fluxes from the catchment, can influence the biological communities in rivers. Depending on the sediment texture and the content of pollutants and/or nutrients also effects like biofilm growth or bloom of toxic cyanobacteria occur.