One of the aims of this paper was to study catchment and watercourse response to extreme events. The study area consisted of Orunski Stream in the northern Polish city of Gdansk. The study period (review of cartographic materials) covered the years 1908 to 2000, while field research covered the period from May 2009 to April 2010. In addition, measurements were performed to study the dynamics of changes occurring in the creek during rainfall events. Examination of the water chemistry changes made two days before heavy rainfall showed a significant increase in sulphates and nitrogen compounds. Additionally, pollutants washed from the basin contributed to a significant deterioration in the water colour. A rainfall of 1.3 mm caused an eleven-fold increase in stream flow, a twofold increase in the water level, and a fourfold increase in the velocity of water molecules. It might appear seem that such a small amount of rainfall would have no major effect on the hydrologic and hydrochemical parameters of a small creek. However, it is the maximum flow rate that is an important variable when assessing potential flood risk levels.