Compared to its surroundings, Lake Balaton is a very young formation developed at the end of the last ice age, almost 20,000 years ago. The mean water level of today's 595 km² and 1.8 km3 lake is 104.5 m above sea level. Prior to the year 1863, the height of the water level was a function of the climate. During the initial high water level, its area could be 900-950 km2, but at the end of the Mesolithic (12,000 to 6,000 BC), even the open water surface could have disappeared due to the warm weather and low rainfall, and the lake basin became a reed-swamp. No such high water-level fluctuation can be detected later. Archaeological data show that, in the first half of the Roman era and in the late migration period, the water level was low, while, due to the climate change, which started in the 13th century, the climate became cooler and wetter, so water level gradually increased. In the Turkish era, during the European Little Ice Age, water level was significantly higher than it is now. The first military surveying published in 1783 also referred to a water level of 108 m. Because of the water management works begun at the beginning of the 19th century, and due to the persistent drought of the 1830s, the water level became lower by the middle of the century than it is today. This allowed building a bridge at the end of the strait of Fenékpuszta in 1839. This bridge connected County Zala and County Somogy. In order to ensure the safety of the Southern Railway (1861), which was very important for the tourism of Lake Balaton, the Sió floodgate was completed in 1863.