Hanayama Dam

The Hasama River is the second largest river in Miyagi Prefecture after the Eai River, and is part of the Kitakami river system. The river source is from Mount Kurikoma and flows southeastward, while also flowing into the Nihasama and Sanhasama River. The primary stream before the confluence point of the Nihasama and Sanhasama rivers was originally named Ichihasama River, and during the dam's construction was still named as such. After this point, the river eventually joins with the Old Kitakami River, while forming a group of small lakes and marshes such as Izunuma and Naganuma as tributaries. 

The dam was constructed at a narrow point where the Hasama River transitions from a mountainous to a plains region. At the time of construction, the dam was located in the then-called Hanayama Village, Kurihara District, which is where the dam gets its name. However, due to the influence from "The Great Heisei Mergers", Hanayama Village merged with other towns and villages in the area and is now Kurihara.

As a part of the Kitakami river system, Hasama and Eai River are used to supply water to the Sendai Plain as tributaries. However, the area has historically encountered frequent damage from flooding, and projects to modify the rivers have been undertaken ever since the period of the Sendai Domain. From these projects; however, side effects have emerged. Because river modification efforts undertaken by the Sendai Domain placed a heavy emphasis on water transportation, work done to the Old Kitakami River caused its floodwaters to be more prone to flow back into the Hasama River.

In 1932, through the work done during the Hasama River Water Improvement Project, a cut-off channel was created in order to correct the meandering nature of the river, resulting in a decrease of its width. This decreased width obstructed the flow of river water, and in 1947's Typhoon Kathleen and 1948's Typhoon Ione, water overflowed from the constricted sections of the river, causing major flood damage to the surrounding agricultural land and its inhabitants.

Meanwhile, in the northern part of the Sendai Plain, the districts of Kurihara and Tome, which provided 620,000 koku to the Sendai domain, largely depended on the Hasama River, leading to frequent water conflicts during droughts. Additionally, as the arable land continued to expand, the traditional method of drawing water directly from the river began to reach its limits.

During this period, the entirety of the Kitakami river system was initially the subject of the Comprehensive River Development Project led by the Ministry of Construction (presently the Ministry of Land, Infrastructure, Transport and Tourism), the Ministry of Agriculture and Forestry (presently the Ministry of Agriculture, Forestry and Fisheries), and the Miyagi and Iwate prefectures. In 1954, the Kitakami River basin received a designation under the name "Kitakami Specific Area Comprehensive Development Plan", with the goal of promoting industrial development in the area, centering on Sendai, through controlled flood control and water utilization. "The 5 Big Dams of the Kitakami River" and the "Eai River Comprehensive Development Project" (see: Naruko Dam, Eai River) were included in these plans. The "First Comprehensive Development Project of the Hasama River", planned by the Miyagi Prefectural Government, was also included in the plans for Hasama River.

Hosokura Mine, established during the Heian Period and located in the Hasama River basin, was a mainstay to the local mining industry until its closure in 1987. In 1934, Mitsubishi Metal Mining (presently Mitsubishi Materials) acquired Hosokura Mine and began mining copper and zinc. However, as time went on, the demand for water and electricity caused by the onsite ore smelting and the increasing number of workers became an issue. In November 1941, the Mitsubishi Kawaguchi No. 1 Power Station (presently Hosokura Metal Mining Co. Kawaguchi No. 1 Power Station) began operation as a conduit type power station in the town of Ichihasama (now Kurihara), supplying the mine with a maximum of 1,550 kW of electricity. However, this amount of electricity still did not suffice, and additional power was required.  

With this historical background in mind, Hanayama Dam entered its planning stages in 1950 as the central project of the "First Comprehensive Development Project of the Hasama River" by the Miyagi Prefectural Government.

Preliminary investigations into where the dam was to be built began in 1950. The current location of the dam was selected for two primary reasons: firstly, the foundation ground was composed of an area consisting of andesite with steep banks on both sides. Secondly, a large man-made lake could be formed to ensure a satisfactory total water storage capacity. In 1952, the dam was designated as a government-subsidized project, and from 1953 an implementation plan study was conducted to determine the dam's specifications.

At the time, 181 households and other facilities were located at the center of the former village of Hanayama, including the town hall, a school, and a clinic. With the construction of Hanayama Dam, these buildings and houses were to be submerged, thus compensation negotiations began with the residents. However, while negotiations were still underway, the prefectural government began construction of temporary facilities to lay a road for the construction of various facilities for the dam, sparking outrage from the local residents. The residents formed the "Hanayama Village Dam Task Force Committee", with a policy of absolute opposition to the dam construction. Following this, compensation negotiations were suspended. While the prefecture halted construction of its temporary facilities and attempted to resume compensation negotiations with the residents, due to the perceived breach of trust the residents felt towards the government, it took over a year before the prefecture was able to convince the residents to begin negotiations once again.

It was not until April 1954 that the residents were satisfied, and compensation negotiations resumed. The prefectural government proceeded to establish the Hanayama Dam Compensation Office under the direct control of the governor, and negotiated with the Hanayama Village Dam Task Force Committee while receiving advice from the advisory Ichihasama River Comprehensive Development Project Task Force Committee. A representative committee of 3 members from the prefectural government and 10 members from the Hanayama Village Dam Task Force Committee was established to hold discussions, and the discussions' results were then sent to the Ichihasama River Comprehensive Project Development Office (on the prefectural government side), the Task Force Committee, the local community advisory board, and the village assembly (on the local residents' side). Both sides engaged in serious discussions, with the residents wagering the survival of the village and the prefectural government vying for the success of the project, which was essential to the success of the Kitakami Specific Area Comprehensive Development Plan.

During the negotiations, a proposal was made to secure replacement land and relocate the village. This proposal was approved, and compensation negotiations concluded on June 30, 1955. Following the negotiations, the Zashu area, located upstream from the former village center, was selected as the replacement land, and construction efforts for the new residences and facilities soon began.

The main body of the dam was completed in 1957, with operations starting in November of the same year.

At a height of 47.8m at the time of completion, the purpose of the dam was fourfold: flood control, unspecified water utilization, irrigation, and hydroelectric power generation. Flood control covers the Hasama River banks from the dam to the confluence of the old Kitakami River, and reduced the estimated high-water discharge of 1,440 tons per second (based on the flooding during Typhoon Kathleen and Typhoon Ione) to 455 tons per second, a cut of 985 tons per second. For irrigation, the maximum rate of supply was 19.6 tons per second to 8,844.5 ha of the northern Sendai Plain.

Purposes related to Hosokura Mine included unspecified water utilization and hydroelectric power generation. In the case of unspecified water use, the amount of water withdrawn to secure water for agricultural use, which is usually the amount of water used under the provisions of customary water rights, is discharged. But at Hanayama Dam, a fixed amount of clean water and industrial use water was provided to Hosokura Mine. 2,592 tons of clean water and 5,184 tons of industrial use water were provided per day. For hydroelectric power, aside from being a water source for the Mitsubishi Kawaguchi No. 1 Power Station (presently Hosokura Metal Mining Co. Kawaguchi No. 1 Power Station), another power station, Mitsubishi Kawaguchi No. 2 Power Station (presently Hosokura Metal Mining Co. Kawaguchi No. 2 Power Station, maximum permissible output: 1,500 kW) was constructed to support Hosokura Mine's operations. In addition to Hanayama Dam, Mitsubishi Metal Mining (presently Mitsubishi Materials) was involved in the development of other dams and related power generation, participating as an electric utility for Moriyoshi Dam (located on the Komata River) in Akita Prefecture.

After the completion of Hanayama Dam, the prefectural government continued to carry out large-scale public works projects in line with its comprehensive development project, with one example being Kurikoma Dam (gravity-type, 62.0 m), completed on the Sanhasama River in 1961. In addition, the company participated in a national agricultural water utilization project led by the Tohoku Agricultural Administration Bureau of the Ministry of Agriculture, Forestry and Fisheries as an administrator for river management. Following the completion of Aratozawa Dam (rock-fill, 74.4 m) on the Nihasama River in 1991 and Oda Dam (rock-fill, 43.5 m) on the Nagasaki River in 2007, management was transferred to the prefectural government Kurihara Regional Dam Management Office. The "Second Hasama Comprehensive Development Project" has been underway since 1971, and the Naganuma Dam (earth dam, 15.3 m) finished construction in 2014 as an auxiliary multi-purpose dam on Naganuma, a natural lake. Many migratory birds rest at these dams during the winter, and the prefecture has embarked on a project to improve the area around the dam lakes in order to disperse the migratory birds in the interest of preventing the spread of avian influenza caused by overcrowding.