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Anthropogenic changes in the suspended,..

tributaries, have been obtained using eąuations for the suspended load balance. Fig. 10 shows the results from these calculations.

The degree of sedimentation within the valley floor shows the river downstream differentation, which is analogous to the sediment delivery from tributaries. Mean values for sedimentation are greatest in the Upper Vistula and are of the order of 10,000 t km ~yr^-1. These values increase downstream to the mouth of the last Carpathian tributary. Directly below this tributary outlet, the sedimentation reaches as much as 45,000 t km^-1yr . Downstream from here, the supply of suspended sediment and sedimentation rates fali to a minimum. Up to 77% of the suspended load which is deposited in the whole Vistula valley floor forms upstream from the Zawichost gauging station. Due to regulation works, intensive overbank sedimentation processes in the Upper Vistula valley floor are significantly influenced by river channel deepening. Downstream, sedimentation decreases markedly.

3. Relationship between sediment deliuery and ouerbank deposition, as eocemplified by the upper stretch ofthe riuer. During the last several decades, the mean duration of inundation of the inter-embankment floodplain, IN, and the average number of flood waves causing inundation, show a pattern in the downstream part of the river which is similar to that for suspended materiał supply from tributaries. Simultaneously, when compared to river channel deepening due to regulation work, the pattern is reversed; this shows that larger Carpathian tributaries provide a huge input of sand to the Vistula which counteracts channel deepening in certain sections. Therefore, due to the shallower channel in this area, the floodplain is inundated morę freąuently The magnitude of absolute and relative overbank sedimentation is also greatest in this area. Therefore, the pattern for the magnitude of sedimentation downstream from the Upper Vistula coincides closely with the pattern for materiał delivery to the river.

PROGNOSIS FOR RIVER LOAD IN THE VISTULA

The progressive decrease in the trends for transportation and sedimentation rates as well as for siltation of existing shallow reservoirs, will be prolonged by building new dams on the tributaries. The location of the dams should be carefully determined in order to reduce river loads to a minimum.

1. Criticism ofthe existing reseruoir distribution within the riuer basin. The deep dam reservoirs on the Carpathian rivers should as far as possible reduce the river load downstream which is further supplied to the Vistula. In this respect, their location in the middle stretches of the tributaries is correct. From a practical point of view, their useful lifetime, which varies between 260 and 1300 years, is sufficient for the Vistula river, to be protected against intensive sediment supply. The role of deep reservoirs located in upper forested stretches of the Carpathian valleys which are slowly being