Foreign river - Gairaikasen

A river that originates in an area with a lot of precipitation and flows into an arid region. Normally, there is no constant water flow in the desert, but rivers with their source in humid regions flow even when there is no precipitation. Once in the desert, the flow rate gradually decreases due to evaporation and infiltration, and small rivers disappear into the desert and become endless rivers or flow into inland salt lakes. Rivers with abundant water flow through the desert and empty into the sea. Irrigation water is easily available in the coastal areas of exotic rivers, so there are many cases of agriculture being carried out along the rivers. The birthplaces of ancient civilizations, such as the Nile, Tigris, and Euphrates, are located on the banks of exotic rivers. Other examples include the Tarim River in China, the Amu Darya and Syr Darya in Central Asia, and the Colorado River in America, which also have farmland along their coasts.

[Shigemi Takayama]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

降水量の多い地域に水源を発して乾燥地域に流れ込む川。普通、砂漠では恒常水流がみられないが、湿潤地域に水源をもつ川は、降水がないときでも流れている。砂漠中に入ると蒸発、浸透などによってしだいに流量は減少し、小さな川は砂漠中で消失して末無し川となるか、内陸塩湖に流入する。水量の豊富な川は砂漠中を貫流して海に注ぐ。外来河川の沿岸地帯は灌漑(かんがい)水を容易に得られるために、流路沿いに農耕が行われている例が多い。ナイル川、チグリス川、ユーフラテス川などの古代文明の発祥地は外来河川の沿岸にある。そのほか、中国のタリム川、中央アジアのアムダリヤやシルダリヤ、アメリカのコロラド川なども沿岸に農地がみられる。

［髙山茂美］

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