Lake Ogawara is a brackish water lake and is the eleventh largest lake in Japan. It has a surface area of 62.3 km² with a mean depth of 11 m and a maximum depth of 25 m. There are four inflowing rivers including the Takase River, all coming from Hakkoda Mountain Range. The only outflowing river is also the Takase River.

The lake was formed from a marine bay some 3,000 years ago by the recession of the sea and the formation of sand bar at its mouth, and has thereafter been gradually transformed into the present configuration. Many remains of human settlements in prehistoric and historic ages are found on hillsides around the lake, showing that the area was more densely inhabited when milder climates prevailed.

Lake Ogawara, is brackish due to the invasion of seawater at times of high tide, but is going to be converted to a freshwater lake by the Lake Ogawara Comprehensive Development Project to serve as sources of drinking, industrial and irrigation water (Q).