This research was focused on the relationship between river discharge and organism drift. It was carried out for three years in a small heavily modified river in Saxony (Germany). The amount and species composition of drifting invertebrates were observed, depending on discharge and flow velocity. A station was installed where the flow velocity was continually measured and drifting organisms were caught with nets. An inventory of the aquatic community (benthic invertebrates) was taken to determine the species living in the river at the research station. The highest drift density measured was 578 organisms per m3 at a flow velocity of 0.90 m s-1 , the mainly drifting organisms were Chironomidae. Different organisms groups started drifting at different flow velocities. Heavy impacts, such as dredging the river and flood waves, affected the aquatic ecosystems and severely changed the aquatic community regarding the number and the diversity. Some of the aquatic invertebrates such as the Anthothecata completely disappeared after dredging. It was found that many different terrestrial organisms were part of the drift. The typical family of soil biota Collembola represented the largest share.
The objective of the study was to evaluate the spatial distribution of peakflow pre-event water contributions and streamwater residence times with emphasis on land use patterns in 38 subcatchments within the 687 km2 large mesoscale transboundary catchment Lužická Nisa. Mean residence times between 8 and 27 months and portions of pre-event water between 10 and 97% on a storm event peakflow were determined, using 18O data in precipitation and streamwater from a weekly monitoring of nearly two years. Only a small tracer variation buffering effect of the lowland tributaries on the main stem was observed, indicating the dominant impact on the mountainous headwaters on the runoff generation. Longest mean streamwater residence times of 27 months were identified in the nearly natural headwaters of the Jizera Mountains, revealing no ambiguous correlation between the catchment area and altitude and the mean resi-dence time of streamwater. Land use control on the pre-event water portions were determined for three land use catego-ries with percentage of urban areas from 0 to 10%, 10 to 20% and more than 20%. The fraction of pre-event water in the first category decreases from 97% to 65% with the increasing percentage of forest from 76% to 100%, revealing that for-ests may provide only a limited infiltration of precipitation due to leaf interception and soil water use for transpiration. Fractions of pre-event water of 39–87% in the second (agricultural catchments) and of 10–35% in the third (urbanized catchments) category increase with percentage of non-urban areas.