L. Forró szerk.: Miscellanea Zoologica Hungarica 10. 1995 (Budapest, 1995)

Tittizer, T., Leuchs, H.; Banning, M.: The consequences of river impoundments for the macrozoobenthos - demonstrated at the example of the River Danube in Germany

Fig. 1. The River Danube. The figure shows the part of the Federal waterway with the finished water retaining structures. (W = weir; 2401.6 = position of weir at rkm 2401.6; 78 = year of damming up) River impoundments and their consequences The impoundment at Geisling, that was completed in 1985, extends from the weir at Danube rkm 2354.0 to the impoundment weir at Regensburg 30 km upstream at the rkm 2381.33. Under mean water conditions the head of the impoundment Geisling is at the upstream end of the city area of Regensburg approximately at rkm 2378. At mean high water it is situated at the upper third of the impoundment around rkm 2374. The construction work at this first impoundment downstream of Regensburg began nearly 20 years ago. Fig. 2 shows that embankment dams were built along both banks. Because of the fact that in the immediate headwater of the weir the water level is about three metres above the terrain, these dams were lined with an internal sealing and all banks were secured by rip-rap rocks. Dredging of the flood plain additionally widened the discharge cross-section. Thus, the width of River Danube has more than doubled to 300 m. Before the impoundment, the mean velocity of flow ranged usually between 0.8 and 1.2 m/s, the damming-up reduced it over long stretches to less than 0.2 m/s; under low-flow conditions it drops in the immediate headwater even to 0.08 m/s (cf. Fig. 3). The drastically modified flow conditions had a similar changing effect on the grain-size distribution in the bed load of the river. Originally, the gravel fraction occasionally with bed-load transport dominated on the bottom of River Danube. In contrast to a free-flowing river, where the sorting of the sediment takes place in a small-scale - often mosaic-like - interlocking manner, large sediment zones can be distinguished in the dammed-up reach. Here, a flow velocity of 20 cm/s constitutes a limit below which the tractive force of the flowing water wanes (Einsele 1960). Consequently, finer material sinks to the bottom and forms larger silt or mud zones. Since the flow in the headwater of the Geisling impoundment is reduced most in comparison with other impoundments, and as the silt content is high, the most severe impacts on the benthic fauna have to be expected here. Moreover, the river continuum is