River-Fish Fauna of the Delta

Human impact through the ages has greatly changed the composition of the fish fauna in the Delta, both qualitatively and quantitatively. Systematic embankment of the Large Rivers was nearly accomplished in the late Middle Ages. However, up to the 18th century, the main channels were meandering and many river islands, floodplain forests and snag habitats were still present. Later, the remnants of flood-plain forests were cleared, whereas snag was removed to facilitate shipping. River regulation and normalisation gained momentum in the 19th century and continues until the present day. The canalisation of the upper Rhine in the 19th century had far-reaching environmental consequences, particularly for the migratory fish species (cf. Chapter 8). The once anastomosing river system with islands, sand and gravel flats, a highly diverse system of various habitats in a dynamic environment, was transformed into a petrified canal with high current velocities (Van den Brink et al., 1996).

The Meuse was a relatively free-flowing river until 1918. In order to facilitate shipping on this rain-fed river, more than 70 weirs have since been built. As a result, the Meuse can be considered to be a chain of basins with long residence times in periods of extreme drought (Van Urk, 1984). The summer-beds of both rivers have become fixed by groynes and dykes which impede meandering and the formation of braided and secondary channels. As a result, the total flood-plain area has become drastically reduced, the river has incised itself into its summer-bed and the river forelands have silted-up. In the 18th and 19th century, dyke bursts occurred regularly during periods of high river discharges in combination with the incidence of ice in the river (Chapter 9). These dyke bursts resulted in tens of metres deep ponds which became filled with river water. Large-scale clay digging and sand and gravel extraction occurred in the 20th century resulting in many new waterbodies (cf. Fig. 13.3). After the great storm-flood disaster of 1953, plans were made to close the large estuaries in the SW Delta. During the execution of the Delta project almost all open connections between the Large Rivers and the North Sea were dammed up, again detrimental to migratory fish (Chapter 10).