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Anthropogenic changes in the suspended..

2. Stage initiated by intensive regulation works on the riuer channel. Cross-section levelling of the inter-embankment zonę started after the regulation works on the river channel had been initiated and was repeated every few years, on average. This has permitted an estimate of the increased sedimentation rates during the first decades of the 20th century to be madę. Later, as discussed previously, the sedimentation rates decreased, and this trend continues. It must be emphasized that the river stretches neighbouring gauging stations where repeated levelling has been madę, represent rather spontaneous overbank sedimentation. Figurę 5 shows the generally decreasing trend in overbank sedimentation rates which have been computed from the data compiled at those gauging stations and river stretches used in the survey. The sedimentation rates refer to the zones which have the most intensive overbank sedimentation, i.e. the overbuilt banks (including the

Fig. 4. Relationships between values of mean annual suspended load supply, XI, to successive stretches of the Vistula (1-15) and ąuantity of suspended load sedimentation, AS, on the Vistula valley floor (= inter-embankment zonę). For numbering of the river stretches see Fig. 1

fast filling inter-groyne basins) and the levees. The rates of sedimentation pertaining to the period before 1920 are not known exactly, but they were probably not less than those after 1920. The data presented here show that the maximum overbank sedimentation rates within the Vistula valley floor occurred during the first years after the completion of river confinement. In fact, river regulation is the most important reason for the acceleration of the transportation and sedimentation rates, especially in the first year period after river-shortening and groyne-construction works had been initiated.