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A. Łajczak

the Lower Vistula, one of only eight on the main river, considerably reduces the suspended load in the river downstream (Fig. 3).

b)    Long-term course of reseruoir siltation and estimation of the useful lifetime of reseruoirs. Downstream effects. Detailed data resulting from repeated reservoir levelling and, also, from the balance of sediment transportation provides evidence of a downward trend in reservoir siltation rates as a result of continuous reservoir shallowing. An intensive erosion of bottom deposits due to density currents and wave-induced sediment resuspension, causes increasing rates of sediment outflow across the dams. The rates of outflow are often independent of incoming sediment supply to the reservoirs. The sediment outflow from deep reservoirs, particularly those being rapidly silted-up (Rożnów Reservoir, for example), increases continuously. By contrast, the sediment outflow from shallow reservoirs, regardless of river size, reaches a more-or-less stabilized value after 5-20 years of reservoir use. The further course of outflow rates has fluctuated round the average amounts of sediment coming into the reservoirs (Fig. 9). Two main stages in the siltation of reservoirs have been recognized (Łajczak 1996):

—    siltation of relatively deep reservoirs, when the bedload is totally trapped by a dam. As a result of reservoir shallowing, the trap efficiency for suspended load decreases progressively to zero. This phase is concurrent with the useful lifetime of an initially deep reservoir, i.e. when its operational properties are fulfilled. The time has been computed using a method previously proposed by the author (Łajczak 1995c, 1996), and, for deep reservoirs in the Carpathians, it varies between 260 and 11 000 years. When compared to the reservoirs situated in cultivated lower parts of the mountains, the reservoirs located in forested catchments are sedimented much morę slowly,

—    much slower siltation, mainly by the bedload, of initially-shallow reservoirs or of much-shallowed, initially-deep reservoirs. The trap efficiency for suspended load fluctuates around zero but, in specific hydrological conditions, the reservoir can become a net exporter of sediments, including large ąuantities of bedload. In the case of a ąuick partial emptying of the upstream associated deep reservoirs during a summer flood event, exceptionally high rates of sediment outflow, which originates from intensive bottom sediment erosion, can occur. The life of shallow reservoirs can be prolonged by flushing and dredging practices. Until now, the flushing of reservoirs in Poland has been employed only infreąuently.

The data presented here demonstrates the much decreased ąuantities of river load in rivers downstream of deep reservoirs, and this reduction is preserved for a long time after the river has been dammed. Shallow reservoirs do not reduce the river load for long periods, but, over short time scales, can introduce considerable fluctuations in sediment transportation rates.

c)    Effects of dredging practices. Dredging practices are used only in certain lowland reservoirs. The most intensive dredging operations started in the Włocławek Reservoir in 1982, with a view to increasing reservoir depth,