Yanba Dam, Gravity dam in Naganohara, Japan

Yanba Dam is a concrete gravity dam on the Agatsuma River in Naganohara, Gunma Prefecture, completed in 2020. Its pale grey face slopes steeply to the river, divided at the crest by concrete piers carrying a green metal walkway and gates, with two dark circular outlets set into the slope.

Planned in 1952 as part of a scheme of ten dams on the Tone River system, the project was frozen because the Agatsuma River was too acidic for dam construction until neutralization works at Kusatsu from 1965. Construction began in 1967 amid strong local opposition; the dam was completed on 31 March 2020 and opened on 1 April 2020, at a total cost of about 532 billion yen.

The reservoir is named Yanba Agatsuma Lake. The name Yanba has several proposed origins, including a hunting ground where arrows were shot (yaba) or a spot with eight pitfalls (yattsu no ana). The dam won the 2024 JSCE Design Award, Excellence Prize.

The dam stands on the Agatsuma River in Kawahara-yu, Naganohara, Gunma Prefecture, reached via the relocated National Route 145 and the Agatsuma Line. The reservoir, Yanba Agatsuma Lake, extends upstream to the confluence with the Shirasawa River. The dam received the 2024 JSCE Design Award, Excellence Prize.

The dam's total cost of about 532 billion yen is the highest ever for a dam in Japan, and 68 years passed between the 1952 plan and the 2020 start of operation. In 2024 the structure received the Japan Society of Civil Engineers Design Award, Excellence Prize.