Lake Varna was formed in a river valley by the raising of sea level near the end of the Pleistocene. Its bottom is covered by a thick alluvium of slime and hydrogen sulphide mud in the deepest parts; there are large deposits of medicinal fango (mineral mud). A number of rivers pour into the lake, including Devnya and Provadiyska that empty near the western shores of Lake Beloslav, which is connected to Lake Varna.