Kokai River

It originates in the Shiona Hills in Nasukarasuyama City in eastern Tochigi Prefecture, flows through Mashiko Town and other areas, flows south, joins the Gogyo River in Chikusei City in western Ibaraki Prefecture, and empties into the Tone River in Tone Town. It is also written as the Kankai River and the Kokai River. It is a first-class river. It has a length of 112 kilometers and a drainage area of ​​1,043 square kilometers. Most of the drainage basin is flat land, and agriculture is centered on rice paddies. The riverbed in the upper and middle reaches is gravel, and the lower reaches are lowland with natural levees, and new rice fields are located along the coast. The rice-growing area downstream in the Ina district of Tsukubamirai City, Ibaraki Prefecture, was created when the Kanto District Magistrate Ina Tadaharu (Hanjuro) developed Yawara Shinden following flood control works during the Keicho era (1596-1615). In Kamihirayanagi in the same district, the birthplace and grave of Mamiya Rinzo are located. River improvements have been made in recent years, and flooding due to overflows has become less frequent. The environmental standard for the river water quality is designated as Type A.

[Mitsue Hirayama]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

栃木県東部、那須烏山(なすからすやま)市の塩那丘陵(えんなきゅうりょう)に源を発し、益子(ましこ)町などを貫流し、南に流れて茨城県西部の筑西(ちくせい)市において五行(ごぎょう)川をあわせ、利根(とね)町で利根川に注ぐ。蚕飼川、子飼川とも書かれた。一級河川。延長112キロメートル、流域面積1043平方キロメートル。流域の大部分は平野にあって、水田中心の農業が行われる。上・中流の河床は砂礫(されき)質、下流部は自然堤防を含む低地をなし、沿岸に新田集落が立地。下流の茨城県つくばみらい市伊奈(いな)地区の米作地帯は、関東郡代伊奈忠治(ただはる)（半十郎）が慶長(けいちょう)年間（1596～1615）の治水工事に続いて、谷原(やわら)新田を開発して出現したもの。同地区の上平柳(かみひらやなぎ)には間宮林蔵(まみやりんぞう)の生家と墓所がある。最近は河川改修が進み、出水による冠水は少なくなった。河川水質の環境基準はA類型に指定されている。

［平山光衛］

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