One of the most extraordinary landscapes of Hungary is Kis-Balaton that can be found near the south-west corner of Keszthely-bay.
Kis-Balaton and Balaton were not distinguished until the end of the 18th century because this area used to be a bay of the lake. The bay played an important role in that Zala could deposit its sediments here. Kis-Balaton lost its filter function gradually by the middle of the 20th century due to the different human interventions. For example: construction of Sió-sluice in 1864, forming of the riverbed of Zala and construction of flood protection dams of Zala river at the beginning of the 20th century. The water-level fluctuation of Balaton decreased significantly after putting the Sió sluice into operation (the fluctuation reached 5-6 m before). First a marshy area covered by reed was formed in the place of the bay where Zala was lost into the swamp. Large part of Kis-Balaton got dry after forming the riverbed and mainly after the construction of the banks. Two small open water surfaces (Vörs and Zalavár) and a bigger (~1400 ha) continuous area covered by reed remained that could filter the water of Zala no longer.
As a consequence of these processes Zala transported its sediments to Keszthely-bay directly including the washed and let in organic material that stayed there due to the special flow conditions. The water quality of Balaton started rapidly degraded as a result of this and due to the continuous economical and technological developments (urbanization, development of agriculture, sewage-disposal). Excess growth of algae began in the lake, seaweed proliferated resulted in eutrophization. Balaton got into such poor condition that it could not meet the needs of bathing in appropriate level.
In order to avoid the ecological disaster the water experts worked out a plan for the construction of “Kis-Balaton Water Protection System”. The basic idea was that Kis-Balaton could filter and treat the sediments and nutrients transported by Zala in the formed marshy area by repeated flooding the former bay swamp area. Processes taking place in Balaton (mainly in Keszthely-bay) are translocated in front of Balaton in the area of the water protection system has to be constructed on the lower section of Zala by this unique water protection system. This solution assures nature like conditions that existed about 200 years ago.