When the lake is at a low level, with a small volume of water, it warms up sooner than most lakes in the region, so people can be seen starting to swim the lake in April while bigger lakes even in lower areas of Switzerland remain rather cool.
As the lake's water level drops, it warms up faster than other lakes in the vicinity, prompting people to start swimming in April. Meanwhile, bigger lakes in lower-lying areas of Switzerland remain cooler until later.