Joint mouth - Joint mouth

...A general term for a structure that crosses an open waterway such as a river, and that raises flowing water and allows it to overflow. Therefore, there is no clear distinction between weirs and dams. However, those that are 15 meters high or higher and those intended to deposit sediment (sabo dams) are called dams, while all other structures are generally called weirs. Weirs have the following purposes: taking in water for irrigation, maintaining the water level and depth for navigation, regulating the flow rate at river diversion points, preventing saltwater intrusion at river mouths, and protecting against high tides. Depending on their purpose, they are classified as intake weirs, diversion weirs, tide-blocking weirs, and tidal defence weirs.

*Some of the terminology that refers to "abutment mouth" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…河川などの開水路を横断して設けられ，流水をせき上げて，その上を越流させる工作物の総称。したがって，せきとダムには明確な区分はない。しかし，高さが15m以上のものや，土砂を堆積させる目的のもの(砂防ダム)はダムと称し，それ以外のものを一般にせきと呼んでいる。せきの目的には，用水の取水，舟運のための水位･水深の確保，河川の分派点における流量調節，河口部における塩水遡上（そじよう）の防止･高潮防御などがあり，その目的に応じて取水ぜき，分水ぜき，潮止めぜき，防潮ぜきなどに区分される。…

※「合せ口」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