Watarase River

A tributary of the Tone River that flows through Tochigi, Gunma, Ibaraki, and Saitama prefectures. It originates in the mountains between Mount Sukai and Mount Koshin in Nikko, Tochigi, and joins the Nitamamotozawa, Kuzozawa, and Mikouchi rivers. It runs roughly southwest from the Watarase, and turns southeast as it enters the plains at Omama. It flows through textile manufacturing areas such as Kiryu in Gunma, Ashikaga, and Sano in Tochigi, and passes through the Watarase Reservoir (also known as the Watarase Reservoir Pond) before joining the Tone River in Koga, Ibaraki (east bank), and Kazo, Saitama (west bank). The total drainage area is approximately 2,621 square kilometers, and the length of the river is 107 kilometers. Bizen Tateyama on the right bank upstream contained copper deposits, which were used to open the Ashio Copper Mine, and in 1907 (Meiji 40), the population of the former Ashio town was over 34,000. Crude copper production peaked at just under 16,000 tons in 1917 (Taisho 6), after which the grade continued to decline, and the mine closed in 1973 (Showa 48). After it was privatized in the Meiji era, problems with smoke pollution and mineral poisoning arose among residents of the river basin, and despite the campaigning of Tanaka Shozo, the village of Yanaka was abandoned.

The upper reaches are used for power generation and mining water. Kusaki Dam, completed in 1976 in Azuma Village, Gunma Prefecture (now Midori City), is a multipurpose dam for flood control, irrigation, power generation, and drinking and industrial water, constructed by the Water Resources Development Public Corporation (now the Water Resources Agency) as part of the Tone River Comprehensive Development Project. It is a gravity concrete dam with an effective water storage capacity of 50.5 million tons. The lower reaches were once prone to flooding. During the Meiji period, the area was hit by 11 floods, the most severe in 1906, with 63 deaths and injuries and 193 houses completely destroyed in Ashikaga County. Water mounds were built to defend against floods, and various ideas were used, such as hanging evacuation boats under the eaves. However, floods have decreased dramatically since the construction of the dam, and the water mounds are being destroyed. The river's environmental standards are classified as Type A in the upper reaches, Type B in the middle reaches, and Type C in the lower reaches. In the lower reaches, centered around the Fujiokamachi district of Tochigi City, Tochigi Prefecture, is the Watarase Reservoir, which covers an area of ​​approximately 33 square kilometers and has a storage capacity of approximately 200 million cubic meters, and regulates flooding of 9,400 cubic meters per second. Construction of the Watarase Reservoir, which will store water by digging further into the reservoir and supply 2.5 cubic meters of municipal water per second to Tokyo and four other prefectures, was carried out as a direct project by the Ministry of Construction (now the Ministry of Land, Infrastructure, Transport and Tourism) and is largely complete.

[Mitsue Hirayama]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

栃木、群馬、茨城、埼玉の各県を流れる、利根川(とねがわ)の支流。栃木県日光(にっこう)市の皇海(すかい)山と、庚申(こうしん)山の間の山中に源を発し、仁田元沢(にたもとさわ)、久蔵沢(くぞうさわ)、神子内(みこうち)川を合し、渡良瀬からほぼ南西に流路をとり、大間々(おおまま)から平野に出るとともに南東方向に転じ、群馬県桐生(きりゅう)市、栃木県足利(あしかが)市、佐野市などの機業地帯を流れ、渡良瀬遊水地（渡良瀬遊水池とも）を経て茨城県古河(こが)市（東岸）、埼玉県加須(かぞ)市（西岸）で利根川に合流する。全流域面積約2621平方キロメートル、流路長107キロメートル。上流右岸側の備前楯(びぜんたて)山に銅鉱床を胚胎(はいたい)し、これを利用した足尾銅山が開かれ、1907年（明治40）には旧足尾町人口3万4000余を数えた。粗銅産出量は1917年（大正6）の約1万6000トン弱をピークにして、その後品位の低下が続き、1973年（昭和48）閉山した。明治時代の民営移管後、煙害と鉱毒問題が流域の住民との間に起こり、田中正造(しょうぞう)の運動にもかかわらず、谷中(やなか)村が廃村となった。

　上流は発電と鉱業用水に利用される。群馬県東(あずま)村（現、みどり市）に1976年竣工(しゅんこう)した草木ダム(くさきだむ)は、水資源開発公団（現、水資源機構）により利根川総合開発事業の一環として築造された洪水調節、灌漑(かんがい)、発電、上水・工水用の多目的ダムである。有効貯水量5050万トンの重力式コンクリートダム。かつて下流部は洪水常襲地であった。明治時代11回の洪水に襲われ、1906年の洪水はもっとも激しく、足利郡においては死傷者63人、全壊家屋193戸であった。洪水に対する防御として水塚(みつか)がつくられ、軒下などに避難用の舟をつり下げるなどのさまざまのくふうが凝らされた。しかし、ダム築造により洪水は激減し、水塚は壊されつつある。河川の環境基準は上流がA、中流がB、下流がC類型に指定されている。下流部、栃木県栃木市藤岡町地区を中心に面積約33平方キロメートル、貯水容量約2億立方メートルの渡良瀬遊水池があり、毎秒9400立方メートルの洪水を調節する。遊水池の一部をさらに掘り込んでここに水を蓄え、東京都ほか4県に毎秒2.5立方メートルの都市用水を供給する渡良瀬貯水池の建設が建設省（現、国土交通省）直轄工事として進められ、おおむね完成した。

［平山光衛］

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