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Anthropogenic changes in the suspended..

channel and this river stretch has been recognized as the main depocentre of bedload within the entire river basin. Further, the floodplain of the Upper Vistula (which has a much reduced width) is the main zonę of overbank deposition in the entire river drainage basin (Łajczak 1995b).

2. Ephemeral deposition effects within the riuer channel due to catastrophic floods. The Carpathian tributaries play a dominant role in the bedload and suspended load supply to the Vistula. Before 1980, catastrophic floods occurred in the river every several years or so and delivered large amounts of sediment, which overbuilt the floodplain and, periodically, aggraded the channel. Rates of aggradation have decreased downstream from the mouth of these tributaries as recorded by repeated river cross-section levelling. Man’s influence on the channel downcutting counteracts the aggradation rates within the Upper Vistula channel over longer time periods, and the large volumes of the bedload deposited there are removed after a few years.

3. Dam construction and reseruoir exploitation. Downstream implications. Since the 1920s, numerous dams have been built within the Vistula drainage basin, eight being located on the main river (Fig. 1). Exploitation of the dams has introduced significant changes in bedload and suspended load transportation:

a) Riuer load trapping by dams. Reservoirs within the Vistula basin are typified by very various geometrie parameters and hydrological properties and, in particular, by different amounts of sediment supply. The intensity of reservoir siltation, dependent on the initial volume and rapidity of water exchange in the reservoirs, and on rates of sediment supply, is greatest in the deep reservoirs in the Carpathians; these are major sediment traps. In the Carpathians, shallow reservoirs associated with upstream deep ones are infilled morę slowly, and are periodically, net exporters of sediment. Large shallow lowland reservoirs on the Vistula and tributaries are rather silted up at ąuite slow rates, as are the shallow reservoirs (which have a rapid water exchange) on the Upper Vistula (Łajczak 1995c).

The rates of river load trapping by dams are well indicated by the "trap efficiency parameter", P, which can be computed using several different methods. The average values of the p parameter of deep reservoirs, calculated for the whole period of their existence and using different computing methods, reach 77-97% (pj), 82-98% (p₂), 93-100% (p₃), and 82-98% (P₅). For shallow reservoirs the P parameter reaches the following rates: 0-53% (Pj), 0-55% (p₂), 0-55% (p₃), and 0-56% (P₅).

Much decreased rates of average transportation loads in the river stretches studied downstream from dams, versus those measured before dam construction, can be demonstrated in only a few cases. Reductions in sediment supply to the Vistula by the Carpathian tributaries due to reservoir construction have reached the following values, for example: the Soła — 90%, the Dunajec — 55%, the San — 10%. Reductions in sediment supply to the river by other tributaries which have only rather shallow reservoirs in them has reached much lower rates. Only the Włocławek Reservoir on