Zalew Sulejowski is an artificially created reservoir with an area of 27 km² located in the Łódź Voivodeship. It was built in the years 1969-1974 as a result of dividing the Pilica River in the village of Smardzewice. It was buit at the 139th kilometer of the Pilica from its mouth, on the border of the then Łódź and Kielce voivodeships1. A concrete and earth dam was built there, 1,200 m long and 16 m high, which contributed to resulting in the creation of a reservoir with a total capacity of 95 million m³.

Lake Sulejowskie was built with Łódź in mind. Plans to create a water reservoir on the Pilica River near Sulejów date back to the mid-1960s. The existing water intakes located in Łódź and its surroundings and in Tomaszów Mazowiecki no longer meet the needs of residents and industry. There was even water rationing. The slogan “Water for Łódź” appeared on banners during the construction of the dam damming the Pilica River and during the clearing of areas intended for flooding.