Izmit Bay, located in the southeastern part of the Marmara Sea has an area of 279 km2. The bay consists of three parts, connected to each other by narrow openings. The bay has a two- layer water stratification and flow system with a halocline / thermocline which separates the lower water layer of Mediterranean origin (35-38 %o) from the upper layer of Black Sea origin (22-28 %o). The thicknesses of the layers change seasonally depending upon the current systems in the area. The major discharges are from the northern part of the Bay.
Izmit watershed is a part of Marmara watershed. Although Dilderesi river and Eastern hannel are the main freshwater inputs to the Bay, both of them carry polluted waters rom surrounding industries, settlements and agricultural area. Dilderesi is 12 km long and carries 70x106 m3/year.