It is a freshwater lake in the eastern part of the Marmara Region, located in the eastern half of a long depression connecting the Adapazarı Plain with the Izmit Gulf Trough. The shores of Sapanca are located in Sakarya province in the east and Kocaeli in the west. Its basin is 252 square kilometers. Its surface area is 47 square kilometers. Its east-west length is 17 kilometers. North-south width is 5 kilometers. Height above sea level is 31 meters. Enderin is 61 meters. The lake takes its name from the district established on the southern shore. It is called Ayan Lake in old Turkish sources.