Dynamika transportu fluwialnego górnej Parsêty jako odbicie funkcjonowania systemu zlewni

Dynamics of fluvial transport of the upper Parsêta River as a response of the catchment system

Andrzej Kostrzewski, Malgorzata Mazurek, Zbigniew Zwoliñski

8. Fluvial transport dynamics

To understand the operation of a contemporary geoecosystem in a young-glacial area, it is necessary not only to define the quality and amount of transported material, but also their variation in space and time. The variability of the fluvial load reflects the dynamics of the processes of soil leaching and erosion in the catchment. Fluvial transport varies over the hydrological year. This results mainly from the distribution and type of precipitation, because it is the factor controlling the amount of water flowing in the catchment, which in turn determines the quantity of material carried away from the catchment system.

The daily concentrations of dissolved and suspended material are presented in Figs. 8.1.1 and 8.1.2, while their monthly characteristics are listed in Table 8.1.1. The daily pattern of dissolved material concentration recorded over the two years indicates four periods of its occurrence in specific intervals. The range of variation in dissolved material concentration was greater for the winter half-years (100 and 70 mg dm-3) than for the summer ones (40 mg dm-3) (Fig. 8.1.3). The reason is the higher rainfall in the winter half-years, and hence more frequent supplying of the river channel with precipitation water. Besides their effect of diluting river water, precipitation water and meltwater are better able to push out and replace old groundwater from the catchment substratum, and this water usually has a higher mineralisation level. In the two-year daily patterns of suspended material concentration, the relation between its variations and the successive half-years is noticeable again (Fig. 8.1.4). Organic particles dominated over mineral ones in the suspended material structure. Generally, the highest concentrations of suspended material were observed during the spring of 1992 and the summer of 1993. The relatively low coefficients of determination (Tables 8.1.2, 8.1.3) for the relationships Cd = f(Q) and Cs = f(Q) prove that variations in the concentration of dissolved and suspended material have a more complex character and depend on more factors than merely the discharge, which is controlled primarily by precipitation.

Flood discharges disturb the calculation of streamflow in the catchment, and hence of the ionic and suspended material flow (Table 8.2.1). The research carried out in the upper ParsÍta catchment so far, as well as the studies conducted in the hydrological years 1992 and 1993, make it possible to distinguish two types of loop for each kind of river material (Figs. 8.2.1, 8.2.2, 8.2.3, 8.2.4). They define the origin of the substances found in the channel, which reflects denudation processes in the catchment system. The relatively low coefficients of determination (Tables 8.1.2, 8.1.3) for the relationships Cd = f(Q) and Cs = f(Q) prove that variations in the concentration of dissolved and suspended material have a more complex character and depend on more factors than merely the discharge, which is controlled primarily by precipitation.

In the two years, 68% of dissolved material and 76% of suspended material left the catchment with flood discharges (57% of the time); the latter load carried away 75% of mineral particles and 76% of organic ones. The ionic flow amounted to 9,440 and 9,664 tonnes, and the suspended flow - 614 and 413 tonnes, respectively. Such a disproportion between the two kinds of flow is characteristic of the catchment under investigation, and of the lakeland zone in general. The rate of chemical denudation can be estimated at 127 t km-2a-1 (4 g km-2s-1) in the hydrological year 1992, and at 131 t km-2a-1 (4,1 g km-2s-1) in 1993, while that of mechanical denudation at 8.3 t km-2a-1 (0.26 g km-2s-1) and 5.6 t km-2a-1 (0.18 g km-2s-1), respectively. The magnitudes quoted (Figs. 8.3.1, 8.3.2) corroborate the claim that chemical denudation dominates over mechanical denudation in the upper ParsÍta catchment (Table 9.4.1). They also confirm that the higher rate of fluvial transport in the winter half-year, indicating more intensive denudation processes in that period, is a stable phenomenon. The dynamics of fluvial transport that emerges from our studies reflects the operation of the catchment's geoecosystem.

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