Being a big and important city, ancient Constantinople (today's Istanbul) was always subject to enemy attacks and sieges in the past. Therefore, the job of water providing and its distribution was always on top of the priority list for the rulers of the city. For example, Romans built the aqueducts, Byzantines built the cisterns, and Ottomans built more dams and distribution centers such as in Taksim district.
Nothing much changed today, water is always top priority of this huge city. There are three main lakes within Istanbul province: Terkos, Kucukcekmece, and Buyukcekmece. These mid-sized lakes were formed when the valleys were
flooded by water many centuries ago. Terkos lake was cut off from the sea therefore it has fresh water, but Kucukcekmece and Buyukcekmece lakes are still connected to the sea therefore these have a little bit salty waters.
It's 50 kilometers to the north of Istanbul on the Black Sea coast, having an area of 25 square kilometers and surrounded by small forests. The lake is fed by Istiranca river so its waters are fresh. It's one of the main fresh water sources of Istanbul using huge pipes connecting to the main water distribution center of the city. The excessive water from the lake is discharged to the sea, if needed. Nearby the lake, there is a small village bearing the same name of the lake. Fresh water fishing and goose hunting is also possible with special regulations at and around Terkos Lake. There are a couple of country style small hotels around it, and attracts many local people especially during summer weekends.
It's 20 kilometers west of the city center on the shores of the Sea of Marmara, having an area of 16 square kilometers. It's fed by Sazlidere, Hadimkoy and Nakkasdere streams and is connected to the sea with a small channel under the breakwater. Kucukcekmece lake has shallow waters (deepest section is about 20 meters) and some fishing is possible, but unfortunately it's facing a dangerous pollution in the last 20 years because of the dense human habitat and uncontrolled
It's 35 kilometers to the southwest of Istanbul city center in the Mimar Sinan neighborhood, near Beylikduzu district next to the Sea of Marmara. The lake has an area of 12 square kilometers with shallow waters (deepest section is about 6 meters), fed by Karasu stream. Buyukcekmece is separated from Marmara Sea by a dam and serve as a reservoir of the city. There is some fishing but lately the lake is endangered by the pollution caused by human settlement and industrial zones.
There are also some small dam lakes around Istanbul, which serve as a water reservoir of the city. Most known ones are Elmali lake, Omerli lake, Alibey lake, Darlik lake, Sazlidere lake, and Isakoy lake. Even though there are
strict regulations against the construction nearby these water sources, there is
still some illegal housing posing a certain degree of threat.
There are several fresh water reservoirs of Istanbul, such as Terkos Lake, Kirkcesme spring, Elmali, Omerli, Alibey, Darlik, Sazlidere and Isakoy dam lakes. These are brought to the main water distribution centers and pumped to the city as drinking and usage water after being filtered and purified by the system.
Other higher quality drinking waters in Istanbul are; Hamidiye water from springs in Kagithane and Kemerburgaz districts, Dertlipinar spring in Ayazaga neighborhood, Kanlikavak spring in Baltalimani neighborhood, Circir, Kestane and Hunkor springs in Sariyer district, Sultan and Kıcatas springs in Buyukdere neighborhood, Kefeli spring in Kirecburnu neighborhood, Kese spring in Alibeykoy district. All of these are in the European side of the city.
There are many spring waters on the Asian side of Istanbul as well; Sirmakes spring, Defneli spring, Goztepe spring, Alemdag spring, Mutevelli spring, Cubuklu spring, Karakulak spring in Beykoz district, Kayisdagi spring waters in Kadikoy district, Tomruk spring in Buyuk Camlica neighborhood, Kucuk Camlica spring in Acibadem neighborhood, Ayazma spring in Yakacik neighborhood, Seker spring etc.
Istanbul doesn't have big rivers within its province borders, the biggest one is Riva river on the Asian side, about 40 kilometers northeast from the city center. Riva is born in Kocaeli province and opens to the Black Sea at the Cayagzi (Riva) village, after 65 kilometers from its origin. Yesilcay (Agva) and Canak streams also end in the Black Sea at the location of Uludere, near Sile town to the northeast. Kurbagalidere stream is about 20 km in Kadikoy district and is heavily polluted, opening to the Sea of Marmara. Goksu and Kucuksu streams (about 16 km) are also polluted and open to the Bosphorus. All of these are on the Asian side of Istanbul.
On the European side of the city, there are Istinye and Buyukdere streams (about 15 km long), Kagithane stream (about 12,5 km) and Alibey stream (about 5 km) both opening to the Golden Horn, Sazlidere stream opening to Kucukcekmece Lake, Karasu stream opening to Buyukcekmece Lake, Istiranca stream opening to Terkos Lake are the most important ones.
There are also some smaller streams such as Elmali, Baltalimani, Belgrad, Sariyer, etc.