One of the objectives of this work was to study the catchment and watercourse response to precipitation. The study area covered the Potok Oruński, located in northern Poland, in the city of Gdansk. The study period using cartographic materials took years 1908–2000, and field research covered the period from May 2009 to April 2010. Additionally, experimental measurements were conducted to observe the dynamics of changes occurring in the brook during rainfall. The rainfall of 1.3 mm caused an eleven fold increase of the flow, a twofold increase of the water level and a fourfold increase in the velocity of the water molecules. It turns out, that the maximum flow rates is an important variable when assessing the potential risks.