İznik is the largest natural lake in the Marmara region and the fifth largest in Türkiye, a freshwater lake sitting in a tectonic hollow. It runs east to west in a long ellipse: thirty-three kilometres end to end, twelve across, with ninety-five kilometres of shoreline and a surface of three hundred and ten square kilometres. Its excess water leaves by the Karsak stream, which carries it fifteen kilometres to the gulf of Gemlik on the Marmara.
The shoreline is where most visitors spend their time — picnic grounds, guesthouses and walking areas, with dense and varied plant cover. Where the feeding streams arrive, the largest being the Karasu in the northeast and the Sölöz in the southwest, they have built small deltas and wide beds of reed. The Karasu is the main one, draining a catchment of two hundred and seventy-three square kilometres.
Away from the shore the lake deepens quickly. Its average depth is thirty metres, and along the southern side runs a trough thirteen kilometres long whose floor, at sixty-five metres, is the deepest point in the lake. The water level shifts with the seasons by fifty to sixty centimetres, and in some years by a full metre.
The temperature tells you what swimming here is like. Surface readings have been taken at 11.5 °C in April, 12 °C in November and 24.5 °C in August, while at forty-five metres the water stays between 8 and 8.5 °C. In summer the difference between top and bottom is severe — around 25 °C at the surface against roughly 9 °C at thirty metres — and the sharpest change falls between ten and twenty metres down. In spring, when snowmelt feeds the lake, that gap closes and the whole column is cold. The lake was declared a protected site in 1990.
When to go
- Climate
- Across Bursa as a whole, monthly mean temperatures run from 5.4 °C in January to 24.6 °C in July; the mean stays between 15 and 25 °C in May, June, July, August, September and October; rainfall is heaviest in December (99 mm) and lightest in August (18 mm). Long-term means for 1927-2025 from the Turkish State Meteorological Service (MGM).
Visiting hours
- Hours
- 08:00 - 17:00, closed Monday
