In January 1990 10 m 3 of washed gravel (grain size 16-50 mm) were introduced in an impounded section of the River High-Rhine to test its potential as a spawning ground for salmonids. The process of infiltration of fine sediments into the gravel and subsequent clogging was monitored for three years. Levels of clogging of the gravel matrix were estimated and ranged from slight to moderately-heavy. The degree of clogging was lower in winter than in summer. Since no bedload transport was observed during the study period, flood events could wash ou~ sediments from the top layer of the gravel bed only. An equilibrium diffusion technique using porewater samplers was employed to measure oxygen concentrations within the interstitial space during the spawning period of Thymallus thymallus (April), Sa/mo tr14ttafario and Oncorhynchus mykiss (December till February). Oxygen concentration decreased with increasing depth and during the course of the study period. Oxygen concentrations measured as a reference in the interstitial of the confluence of the river Glatt were considerably lower than those of the new gravel bed. A wide range of 0 2 concentrations was found in winter 1991/92 and in spring 1992. This could be explained by the heterogeneous microstructure of the substrate. Successful embryonic and larval development of grayling in the gravel bed was observed in spring 1991 and 1992. However, no eggs or larvae of brown trout or rainbow trout were found. Habitat restoration projects for salmonids in impoundments of the High-Rhine are critically discussed.
The meander-type fish pass is a slotted fishway characterised by the exclusive use of rounded and smooth components. Due to the specific geometry and arrangement of the basins, which differ considerably from the conventional vertical slot pass, there are significantly different hydraulic conditions. The water flow is guided along the basin walls by means of a dominant main current while the water body is much calmer towards the centre of the basin, where very low flow velocities are found. A detailed assessment of the functionality is currently impossible due to the small number of surveys of fish passage hitherto carried out. However, considerable potential is indicated by the high passage rates at some sites and the lack of selectivity with regard to species and small fish, together with the design advantages and the adaptation possibilities after construction work is completed. In this study, we offer some recommendations for dimensioning to enable this potential to be exploited. These are oriented around recommendations for conventional vertical slot passes regarding basin size, flow depth and slot width. Due to the great potential of the meander-type fish pass, it is desirable to construct still more individual pilot passes to implement comprehensive surveys of fish passage on a sound methodological basis.
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