Lake Furukawanuma

Lake Furukawanuma (古川沼, Furukawanuma) was a lake that existed in Rikuzentakata, Iwate, Japan until March 11, 2011. When the 2011 Tohoku earthquake struck, the resulting tsunami destroyed the sand wall that separated the lake from the sea. The lake's western edge thus became part of the new coastline, and the lake became part of the sea.

Rikuzentakada is the largest alluvial plain along the ria of the Sanriku Coast. The plain started to form at the last stage of the ice age when the sea level rose. Sand was carried from the Kesen River into Hirota Bay. About 7500 years ago, the surface area of the bay grew, thus the present plain was under water. Mud deposited in the Furukawanuma Lake was formed by the formation of sand hills about 1,000 years ago. There was a 200 to 300 metres (660 to 980 ft) wide deposition of sand blocking part of the Hirota Bay. Mud was deposited in Hirota Bay which became deeper. Pine trees were planted along the seashore in the Edo period. The Sanriku Coast has many ria coasts which have frequently been shaped by the actions of tsunami. In 1835, during the Tenpo era, a tsunami destroyed the pine trees, but the trees acted as a barrier and the city of Rikuzen Takata was saved from the full impact of the tsunami.

The Kawara River flowed into the lake from the north-west, while the Koizumi River flowed from the east. The rivers contained particulates that fell to the lake bed beneath. The Kesen River flowed into the lake when there were floods, and the incoming fresh water mixed with sea water. A lock was installed to facilitate the transport of boats to and from the ocean, but it did not block all the water from the sea.

The damage of the city by the tsunamis of 1896 and 1933 was comparatively small because of the pine trees which served as a tide-water control forest. The Chile tsunami (or 1960 Valdivia earthquake) mauled the city of Sanriku Takada. The tsunami came over the sand hills and the 3 metres (9.8 ft) high dike, built in 1908, and over the dike of the northern part of the lake into the Kawara River. Low areas of the city were flooded. A 140 metres (460 ft) segment of the dike was destroyed, which formed a water lane for over 200 metres (660 ft) connecting the lake and the sea. Through the efforts of 1,500 soldiers of the Imperial Japanese Army, a dike was made between the lake and the lake.

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