Reservoir - Yusuichi

An artificially created reservoir that temporarily stores floodwaters in lakes, marshes, flat land, or river channels along rivers. In the past, floodwaters were diverted from rivers into flood control reservoirs without levees or with discontinuous levees, but in recent years, overflow levees, which are portions of levees deliberately lowered, have been used. A flood control reservoir that is completely separated from the river channel by a levee like this is called a flood regulation reservoir. In contrast, a river channel flood control reservoir is one in which a cross levee is installed in the river channel, almost at a right angle from the main levee toward the center of the river, slowing the flow and storing water within the river channel. The water stored in a flood control reservoir is returned to the river by a sluice gate and drainage pump as the main river recedes.

In Japan, the Watarase Reservoir (often written as Watarase Reservoir) of the Tone River system is particularly large, with a capacity comparable to that of Ogouchi Dam's Lake Okutama (Tokyo). Cambodia's Tonle Sap Lake is a natural reservoir on the Mekong River, and in the rainy season, the river water flows back into the lake, making its area three times that of the dry season.

[Isamu Kayane]

"The Watarase Reservoir: Protecting its History and Nature" (ed. Tone River Basin Residents Council for the Protection of the Watarase Reservoir, 1991, Suizaisha)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

河川沿いにある湖や湿地、平地または河道内に一時的に洪水時の水を貯留させる人為的に設けられた池。以前は無堤防あるいは霞堤(かすみてい)（不連続に設けられた堤防）によって洪水を河川から遊水池に導いたが、近年は堤防の一部をわざと低くした越流堤(えつりゅうてい)によっている。このように堤防で河道と完全に分けられた遊水池を洪水調節池とよぶ。これに対して、河道の中に本堤からほぼ直角に川の中央に向かって横堤(よこてい)を設けるなどして、河道内で流れを緩めて貯留する場合を河道遊水池という。洪水調節池に貯留した水は、本川が減水するにつれて樋(ひ)門および排水機によって河川へ戻す。

　日本では、利根川(とねがわ)水系の渡良瀬(わたらせ)遊水池（渡良瀬遊水地の表記が多い）がとくに規模が大きく、その容量は小河内(おごうち)ダムの奥多摩湖（東京都）に匹敵する。カンボジアのトンレ・サップ湖はメコン川の天然の遊水池で、雨期になると川水が湖へ逆流し、面積が乾期の3倍にもなる。

［榧根　勇］

『渡良瀬遊水池を守る利根川流域住民協議会編『渡良瀬遊水池　その歴史と自然を守るために』（1991・随想舎）』

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