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Structure of the solute yield in the Vistula..

here the lowest in the whole Vistula river basin and amounts to between 10-12 and 20-25 t km^y^-1. It coresponds to Iow indices for solute yield, and also to flatland relief determining a limited thickness of leached layers composed mainly of sandy deposits (including glacial ones of older glaciations, i.e. intensely weathered).

The region of the North Polish Lakę District is rather differentiated; among the analyzed catchments the most representative are those of the Brda, Wda and Drwęca rivers. They are characterized by considerably higher solute yield (in the interval 30—80 t km^y^-1) than those in the lowland belt. This is connected with greater indices of speciflc river dis-charges and with widely spread, unweathered, carbonate glacial sediments of the last glaciation. The intensity of Chemical denudation is mostly in the interval 20-30 tkm^_2y~\ and it constitutes about 50% or morę of the total solute yield.

From the review of regional differentiation, it appears that the structure of the solute yield from rural areas corresponds to the natural conditions of the particular catchments, i.e. hydroclimatic and geological-morphological ones. This problem was discussed in our previous papers (Maruszczak and Wilgat 1993, 1996), in which we stressed that the structure of river solute yield could be treated as an indirect differentiation index of geosystems.

BALANCE FOR RIVER SOLUTE YIELD IN THE VISTULA RIVER BASIN

The balance previously published (Maruszczak 1990) was calculated on the basis of few data relating to the second part of the 70’s. The index of total solute yield was then calculated in a simplified way, multiplying the mean discharges of many years at Tczew and mean solute concentration

measured in the years 1974-1977 at Kieżmark. This index amounted to

—2 —1

50.9 t km y . In this paper we calculated the ąuantities of solute yield by adding monthly values for transport. The thus-calculated index of the total solute yield from the Vistula river basin in the years 1976-1985 amounted to 65 t km^_2y . So, the increase in comparison with the index calculated previously for the second part of the 70’s is very high (20%). It can be explained by reference to increasing pollution of Poland’s rivers in the 80’s. Of great importance here was a very large increase in pollution of the Little Vistula river connected with the development of new mines in the Rybnik Coal Basin. In the second part of the 70’s the mean index for solute yield from this catchment amounted to 310 t km^-2y^{- 1}, and in the first part of the 80’s to as much as 872 t km^-2y^-1 (see Table 1).

The balance eąuation for solute yield from the Vistula river basin in the years 1976—1985 is as follows:

SYt = SYp + SYch + SYfs + SYmis + SYd

where: