Studies of the water circulation and leaching are usually carried out in mountainous and upland areas with thin layers of skeletal soil and the river discharge controlled by the overland flow, throughflow, or flow from shallow groundwater. The results presented in this study are representative of postglacial environments with highly variable geological structure, permeable deposits, thick soil profiles, and high retentiveness of catchments. These conditions alter fundamentally the time and pathways of water circulation, the spatial and temporal variability of fluvial transport, and also the intensity of denudation.

The various forms of feeding a river channel, together with the availability of dissolved substances, are of fundamental significance for the time of material supply from the catchment to the river channel as well as for the magnitude and dynamics of transport. The aim of the present study was to gain an insights into the nature and dynamics of the flow of dissolved substances in an environmentally heterogeneous postglacial catchment.

The field research was conducted in a small catchment drained by the Kluda River, Poland. This is a part of the upper Parseta hydrographic system, which is considered to be representative of the postglacial lakeland zone of West Pomerania and the Polish Plain. The study is based on hydrological and hydrochemical data from the period 1990 - 1993. Hydrological measurements carried out at a water-gauging station located on the 6.5th kilometre of the Kluda. The variability of solute transport was determined on the basis of 83 periodic measurements taken in the water-gauging profile closing the Kluda catchment. Temporal changes in the concentrations of selected chemical parameters of the Kluda river were used as natural geochemical indicators of variability of chemical denudation processes in a postglacial catchment in the temperate zone.

The periodic observations of the physico-chemical parameters of the stream water show low variability of total mineralisation and the concentration of ionised silica as well as calcium, magnesium and bicarbonate ions. The slight changes in the concentrations of compounds dissolved in the stream water indicate that in the range of discharge registered during the research it is fed predominantly by groundwater. The high permeability of deposits, retentivity of the substratum and mineral-organic deposits with a high groundwater level make it easy to "push out" the water present in the catchment before precipitation, i.e. the "old" water.

In a catchment of low relief, with deep soils and important role of groundwater flow like the Kluda catchment, only a small proportion of the variance in concentration may be explained by discharge. The high retentiveness of the catchment system explains the absence of direct links between the magnitude of precipitation, the discharge, and water chemistry in the river channel. The concentration - discharge relationships over long periods may be also obscured by seasonal fluctuations due to biological or geochemical processes. Other factors that may control those relations include:

• seasonal variations in ion concentrations resulting from their role in the biological cycle;
  
• the chemical composition of precipitation water and the activation of dry deposition;
  
• the feeding by water from the overland flow and throughflow, since it is enriched with compounds coming from short-term and seasonal washout of salts;

effects of hysteresis which often explain why the connection between the ion concentration and the discharge is absent or only loose, especially when we deal with seasonal measurements that do not embrace entire flood events.