A Key Landmark on the Naka River
The Naka River is one of Tokushima Prefecture’s major waterways. Rising in the Tsurugi mountain range near Mount Jirogyu, it winds through steep valleys and scenic gorges, including Boke Gorge and the Washiki Line, before flowing into the Kii Channel near Anan City.
Nagayasuguchi Dam stands just downstream from the confluence of the Naka River and the Sakashu-Kito River. It forms part of a group of three major dams in the upper Naka River basin, collectively known as the Naka River Upper Dam Group. Upstream is Kominono Dam, a 62.5-meter-high concrete arch dam, while Kawaguchi Dam, a 30-meter-high concrete gravity dam, lies downstream.
The name Nagayasuguchi comes from its location at the entrance to the Nagayasu district. When the dam was completed, the area belonged to Kaminaka Town in Naka District. Following municipal mergers during Japan’s Heisei-era consolidation, the area became part of present-day Naka Town.
Why the Dam Was Built
The Naka River basin receives substantial rainfall, averaging approximately 3,200 millimeters annually. Located in a region frequently affected by typhoons, the basin has a history of severe flooding. The river’s steep gradient and fast-flowing waters can cause levels to rise rapidly, threatening communities and farmland downstream.
River improvement projects began in 1929 under the national government. However, the devastating floods caused by Typhoon Jane in 1950 exceeded the previously anticipated flood discharge, reaching approximately 9,000 cubic meters per second. This disaster prompted a reassessment of flood-control planning and strengthened the case for constructing a major dam to regulate river flows.
Water demand was another important consideration. Agricultural expansion created a need for reliable irrigation, while paper mills in Anan required a stable industrial water supply. Tokushima Prefecture was also pursuing plans to develop port facilities and industrial areas, making hydroelectric power an important part of the region’s development strategy.
Following the enactment of the National Comprehensive Development Act in 1950, the Naka River basin became the focus of an integrated development plan inspired by the Tennessee Valley Authority model in the United States. Nagayasuguchi Dam was selected as the first major project under Tokushima Prefecture’s Naka River Comprehensive Development Plan.
Flood Control and Water Management
Flood control remains one of the dam’s principal functions. The original planning aimed to reduce the design flood discharge from 6,400 to 5,400 cubic meters per second, helping moderate floodwaters before they reach downstream communities. Six large gates are installed to manage the flow of water through the dam.
The dam also supports agriculture and industry across southern Tokushima. It supplies irrigation water to approximately 7,300 hectares of farmland in Anan City, Komatsushima City, and Naka District, with a maximum planned supply rate of 30 cubic meters per second. It also helps provide industrial water under water rights held by major paper manufacturers, including Oji Paper and Nippon Paper Industries.
Hydroelectric Power Generation
Nagayasuguchi Dam contributes to the region’s energy supply through Hino-tani Power Station downstream. The station has a generating capacity of 62,000 kilowatts and uses Francis turbines to convert the energy of flowing water into electricity. This hydroelectric power supports the industrial and economic activities of the surrounding region.
Because the dam serves several purposes, its operation requires a careful balance between flood protection, water supply, and power generation. Reservoir levels and water releases are therefore important not only for river management but also for agriculture and industry throughout southern Tokushima.
The Human Story Behind Construction
The construction of Nagayasuguchi Dam brought significant changes to local communities. A total of 106 households in the areas that are now Naka Town and the former Kisawa Village were affected by inundation. Group negotiations over compensation began in 1953, but disagreements over compensation standards led to a breakdown in talks the following year. Individual negotiations continued, with some residents maintaining strong opposition until an agreement was eventually reached through mediation by the Tokushima Prefectural Assembly.
The project also affected traditional industries. Fishing was an important livelihood along the Naka River, and the regional fisheries federation initially opposed the construction. Negotiations eventually resolved the dispute. In addition, the dam made the traditional transport of timber by river rafts impossible, affecting 1,037 raft-transport workers. Compensation was provided, alongside measures such as road construction and support for the forestry industry.
These changes illustrate how large-scale infrastructure projects can reshape both the natural environment and the lives of the people who depend on it.
Discovering Nagayasuguchi Dam
For visitors exploring the mountain country of Naka Town, Nagayasuguchi Dam offers an opportunity to appreciate the scale of modern civil engineering within a dramatic natural setting. The surrounding river valleys and forested mountains reveal the rugged landscape that has shaped the Naka River and its communities.
The dam is also a reminder of the close relationship between Tokushima’s natural environment and its economy. Its flood-control system protects downstream areas, its reservoir supports essential water supplies, and its role in hydroelectric generation contributes to regional industry. Together, these functions make Nagayasuguchi Dam one of the most important infrastructure landmarks in southern Tokushima.