Lake Shumarinai is the only man-made lake in Hokkaido, Japan. It was formed by the construction of Uryu Dam in the upstream of Uryu River to serve as a reservoir for hydroelectric power generation. The lake is called generally Shumarinai-ko, but it is also called formally the First Uryu Reservoir. The dam construction was initiated in 1937 and completed in 1943 by Hokkaido Electric Company. The lake now supplies irrigation water at an average rate of 9.73 m³ sec^-1 only for summer season from May to August together with hydroelectric power. It also plays an important role to prevent flood in the downstream area at the time of spring thaw.

The lake is located about 20 km west of Nayoro City. With 13 islands, the lake is irregularly arborescent in shape. The area surrounding the lake is designated as a prefectural natural park for preserving wildlife and other natural resources. Not only hunting and capture of animals but also deforestation are strictly prohibited.

The climate of Lake Shumarinai is characterized by severe, long winter and dry, short summer. The lake freezes regularly from late autumn to late spring. Horokanai Town located on the shore of this lake is so famous as one of the coldest places in Japan; an air temperature of -41.2deg C was recorded in the town in 1978.

The catchment area of reservoir (312.1 km²) is sparsely populated and largely covered by dense forest, containing only a small part of agricultural land. The water has remained oligotrophic, chlorophyll and total phosphorus concentration being very low (Q, 1).