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T. Sokołowski

profile. However, the raeasured maximum discharge was 4400 m³/s in 1960 while the estimated one was 6100 m³/s. These values were 4660 m% and 6300 m³/s in 1970, respectively.

The sediment load transported by the Vistula is fairly large. The total sediment load transported in the period 1971—1978 varied from    ⁸⁰

369 000 to 602 000 tons/yr. Mean turbidity and sediment yield exhibit similar tendencies of changes as these are the case of water stages and    60

discharge in an annual scalę (Fig. 4). The largest sediment transportation is during the summer freshets. For example, the flood in August 1972    _w

carried 43°/a of the total annual sediment load.

PRESENT-DAY VISTULA CHANNEL AND ITS CHANGES DURING THE LAST 300 YEARS

The Vistula was a meandering river still in the 17th century as in-dicated by the plan of Michałowski (1678) which included some villages to the south of Nowy Korczyn. The Vistula is depicted as a straight river on the later topographic maps of the 18th and the beginning of the 19th centuries (maps of Mieg and Waldau, and of von Haldensfeld). The sinu-    J

osity of the river on those maps is 1.28, thus it is almost identical to    J

that of the present-day channel. When interpolating the changes of the river sinuosity one can assume, therefore, that the Vistula became    i

a straight river generally in the 18th century. However, the tendencies to the earlier changes of the channel sinuosity cannot be eliminated what is    ]

evidenced by straight undercuttings preserved to the west of Szczucin (Fig. 1). The topographic surveyings mentioned above have been madę before the channelization. Hence, the change of the channel naturę has resulted from an artificial river shortening freąuently performed during a river management (Kędzior 1929).

Interchannel bars as well as point bars were fairly well pronounced within the channel of the 19th century. These bars concentrated morę abundantly directly downstream of the Dunajec mouth. The channel width was most freąuently 300—500 m at that time and sporadically reached 900 m. Therefore, the Vistula had a tendency to braiding.

The bars, predominantly the sandy ones, occur in the central parts of the stream current and close to the banks within the present-day, straight channel, the latter in a form of gentle bends. The presence of the bars is strongly related to that of groynes. Most of the bars, especial-ly of the interchannel ones are permamently under the water, even during the Iow water stages.

The erosional tendencies of the river channels have been suggested many times in the case of the upper Vistula and its tributaries (Punzet 1981; Klimek 1983; Trafas 1975; Alexandrowicz et aL 1981). The per-mament lowering of the Iow water stages is considered to be a measure