This paper attempts to define the dynamics of the surface storage in water circulation in brackish marshes located in the contact zone of the land and sea. This study estimated the quantity of water stored in the area of the Beka reserve during mapping between December 2011 and December 2013. The study area is characterized by the simultaneous influence of marine and fresh waters. The hydrographic situations observed in the area of the Beka reserve are a momentary picture of the surface storage. The maximum retention periods of surface water on the Beka reserve include nearly 40% of the marsh area. The main source of supply of such large quantities of water is not only the atmospheric supply, but also the seawater inflow, particularly often observed during the autumn–winter storms in the Baltic Sea, as well as other periodic flooding of water from the rivers, canals and ditches located within the reserve. At other times, the area occupied by the surface water is, on average, from ca. 2% to nearly 12%. Only in the summer periods is a decrease in the surface (below 1%) observed due to the strong evapotranspiration in the study area.