Hydraulic connection between Vistula and Curonian lagoons of the Baltic Sea

Potentially favourable conditions for overflows of water between the Vistula and Curonian lagoons via two branches of the Pregolya River were studied using water level data (2002–2007) at the hydrological stations located in lagoons and the river. The episodes (of two days and longer) of the ‘reversed slope’, when the water level in the Vistula Lagoon (7 cases) and the Curonian Lagoon (41 cases) was higher than in the river streams connecting them, were found. This confirms the theoretical possibility of water overflow between the Vistula and Curonian lagoons, back and forth. Some unusual cases (of two days and longer) when the ‘reverse slope’ of water level existed simultaneously in both recipient bodies (Vistula and Curonian lagoons) were also found. The analysis showed that wind conditions were not the single determining factor. Direct measurements of near-bottom currents showed that backward water flow appeared in both branches. Backward water flow developed mainly in the low parts of river branches, i.e. closer to the lagoons. Time delay in water flow variations and the appearance of backward water flow between downstream and upstream points was 2–7 hours, or about 4 hours on average. The signal of backward current in one branch penetrated upstream not only the other branch (correlation coefficient was about 0.64 and 0.71 for 1-h and 12-h running averaged series, respectively) but further upstream the mainstream of the Pregolya River.