The quality of surface water is shaped by a number of processes taking place across the catchment. The dominant role in shaping changes in physico-chemical properties of water play hydrochemical conditions. Detailed characteristics of the aquatic environment is an important aspect in assessing the variability of temporary surface water quality, and the information is necessary for sustainable water management. Therefore, the aim of this study was to investigate the seasonal variability of the chemical composition of water of a small stream in the agricultural landscape in north-eastern Poland. The study included analysis of selected physico-chemical properties of water per year, performing factor analysis in order to identify the processes that shape seasonal changes and the calculation of indicators Markham (index of seasonality and time of concentration). The results found that the physico-chemical properties of water examined watercourse been subject seasonal fluctuations. It indicates the relationship between the content of the individual components dissolved and the size of flow. It was found that in conditions of Polish north-eastern intensive washing and increased levels of nutrients were observed during high water caused by melting snow and rainfall in winter and early spring. Therefore, this period can be critical for the quality of surface water in the agricultural landscape.