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A. ajczak

large-scale morphological mapping of selected stretches of the floodplain, analysis of old topographic maps, and from drilling of infilled meanders and inter-groyne basins.

The rates of reservoir siltation and of annual sediment output from the reservoirs’have also been computed using the input-output formula of the author (Łajczak 1995b). The trap efficiency, P(%), of reservoirs for suspended load has been derived from Brune’s (Pj), Drozd’s (P₂) and Hartung’s (P₃) formulae and by means of the balance eąuation of load transport (P₄). The real trap efficiency of reservoirs (P₅) (for total load), has also been estimated. The average rates of deposition, AS, have been extrapolated for longer time periods. On this basis, using the author’s own computingmethodology (Łajczak 1995c), it becomes possible to calculate the useful lifetime of any particular reservoir.

RESULTS

ESTIMATING THE GENERAL TRENDS IN TRANSPORTATION AND SEDIMENTATION RATES

The current rates of suspended matter sedimentation, AS, in successive stretches of the inter-embankment zonę of the Vistula (limited by gauging stations with suspended load measurements) are exactly dependent on the amount of sediment supply to the river stretch from upstream and from tributaries (Fig. 4). Regression lines were computed on the basis of mean 5-year values from a 45-year measurement period. These results permit at least a rough correlation to be madę between the transportation and sedimentation rates (within the inter-embankment zonę) which had occurred prior to the measurement of suspended load which was initiated in the river.

Three main phases in the long-term history of transportation and sedimentation by the river may be distinguished (N.B. these are directly related to man’s impact on fluvial processes).

1. Stage prior to generał regulation works on the riuer channel. During this first phase, increased rates of transportation and sedimentation by the river occurred. Prior to the end of the Middle Ages significant aggradation occurred on the valley floor. Łater, both climatic changes and morę intensive cultivation accelerated transportation and sedimentation rates in the river valley. The wide and shallow channel thus created became bordered by wide silt-sand levees. Non-active channels and oxbow lakes neighbouring the active channel were partially or completely infilled. This trend towards accelerated sedimentation continued until well into the second half of the 19th century. Falkowski (1975, 1982) showed that the rates of overbank sedimentation increased in several places along the Middle Vistula valley floor prior to the generał introduction of regulation works on the river channel.