The largest lake in Tallinn is Lake Ülemiste (), which serves as the main source of the city's drinking water. Lake Harku is the second largest lake within the borders of Tallinn and its area is . Tallinn does not lie on a major river. The only significant river in Tallinn nowadays is Pirita, in the eponymous Pirita city district.  Historically, the small Härjapea river flowed from Lake Ülemiste through the town into the sea, but the river was diverted into underground sewerage system in the 1930s and has since completely disappeared from the cityscape. References to it still remain in the street names Jõe (from ''jõgi'', river) and Kivisilla (from ''kivi sild'', stone bridge).