Akamanuma Pond

...The hilly Boso Peninsula and Miura Peninsula are higher on the Pacific side, and most of the rivers that flow through these two peninsulas flow north or northwest into Tokyo Bay and Sagami Bay. The Kanto region is a large, shallow basin, and its lowest part is near Koga City, where the Watarase River joins the Tone River, and Akamanuma Lake serves as a flood control reservoir. It was said that Edo would be in danger if the levees at Sekijuku, the junction with the Edo River, were to break, and flood control works such as river channel rerouting have been carried out repeatedly from the Edo period to the present day.

*Some of the terminology that refers to "Akamanuma Swamp" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…丘陵の広い房総半島，三浦半島では太平洋側が高く，両半島を流れる河川の多くは，北または北西へ流れて東京湾，相模湾へ注ぐ。関東地方は大きく浅い盆地となり，その最も低い部分は，渡良瀬川が利根川本流に合流する古河（こが）市付近で，赤麻沼が洪水調整池となっていた。江戸川との分流点，関宿の堤防が切れると江戸が危いといわれ，河道の付替えなど治水工事は江戸時代から現代までくり返し施工されてきた。…

※「赤麻沼」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報