Yonai Water Purification Plant

The Yonai Water Purification Plant

The Yonai Water Purification Plant (米内浄水場, Yonai Jōsuijō) in Kamiyonai, Morioka City, Iwate Prefecture, is the city's first waterworks, completed in 1934; seven structures of its original slow sand filtration line and its former administration office, now the Waterworks Memorial Hall, were listed as Registered Tangible Cultural Properties on 23 August 1999. The eight carry consecutive registration numbers, 03-0012 to 03-0019, and all were listed for contributing to the historic landscape of the country.

Read in the order the water travels, the eight are a receiving well, a sedimentation basin, a three-bed slow sand filter, a small control house beside each filter bed, an octagonal outlet well near the gate and the memorial hall. All belong to the Morioka City Waterworks Manager, the official who runs the municipal water utility. The Yonai Water Purification Plant was never retired: it still treats water from the Yonai River, now with both slow sand filtration and rapid filtration, and has a capacity of 32,450 cubic meters a day.

In May 1985 the plant was named one of Japan's 100 notable modern waterworks. Morioka City added its own recognition on 10 October 2018, listing the plant as Landscape Important Building No. 19.

The last prefectural capital without piped water

Morioka grew up where the Kitakami, Nakatsu and Shizukuishi rivers meet, and for centuries its people drew water from shallow wells. The city waterworks bureau's history recounts that when Iwate Prefecture tested 2,545 wells in the city in November 1925, 622 were judged unfit to drink, and only 91 of the rest gave water better than the Yonai River that would later become the source.

Across the six prefectures of Tohoku, Akita had opened a public water supply in 1907, and the other capitals followed by 1925, when Fukushima's system was completed. That left Morioka as the only prefectural capital in the region without one.

Mayor Nakamura Kenzo put survey money into the 1929 budget, and that October Kawaguchi Kyosuke, an engineer of the Home Ministry, inspected three candidate rivers. He favored the Yonai for its volume, its clean water and the roughly 2,000 hectares of prefectural forest that fed it. City engineers led by Ota Yutaro, head of the planning section, finished the design in 1931: water would run by gravity from the intake to the plant and on to the distribution reservoir.

Full-scale work began in May 1933, with crews hired from the more experienced Kanto and Kansai regions for the plant and reservoir and local firms hauling materials and laying pipe. The works were finished in August 1934, an opening ceremony followed on 23 November, and homes received piped water from 1 December 1934.

Layout of the Yonai Water Purification Plant

The Agency for Cultural Affairs descriptions, read together, trace the water's path. Raw water is taken from the Yonai River about 1 kilometer upstream, passes a grit chamber, and arrives at the receiving well on the plant's eastern edge. After metering, it flows into the sedimentation basin just to the north. Both sit on a raised terrace on the east side of the grounds.

South of the sedimentation basin lies the slow sand filter, 90 by 37 meters, split by partition walls into three beds. A small control house for the valves stands at the middle of each bed's southern edge, numbered 1 to 3 from east to west; No. 3 is closest to the Yonai River.

The main gate faces Kami-Yonai Station on the JR Yamada Line. Inside, the octagonal outlet well house is on the left, and the filter beds with the No. 3 control house open out on the right. The timber Waterworks Memorial Hall stands toward the southeast corner, where it overlooks both the sedimentation basin and the filters.

The control houses and the outlet well house are reinforced concrete but share a decorative vocabulary of Oya stone trim and round windows, which ties the grounds together visually. Morioka City's landscape listing stresses that the memorial hall, the outlet well house and the filter beds are still working together and so keep the city's water history in daily use.

The double-flowered weeping higan cherries on the grounds belong to the same story. According to the waterworks bureau they were planted when the plant was completed, and the group is protected by the city as a landscape important tree stand.

Visiting the Yonai Water Purification Plant

Because it is a working water plant, the grounds are normally closed to the public. The chance to walk inside comes each spring, when the bureau opens the site while the cherries bloom. In 2026 the opening ran for 15 days, from 22 April to 6 May, from 9:00 a.m. to 4:30 p.m., with the memorial hall open until 4:00 p.m. Dates can shift with the blossoms and are announced on the bureau's website. Eating, smoking, sitting on mats, bringing pets and flying drones are not allowed during the opening.

For the Waterworks Memorial Hall itself, the bureau lists opening on weekdays from 16 April to 30 November, 9:30 a.m. to 4:30 p.m., closed on Saturdays, Sundays and public holidays, with free admission and a request that visitors apply in advance. Enquiries go to the Yonai plant (tel. 019-667-2280). The bureau's guided waterworks tours, held around Water Week in early June, sometimes include the plant as well.

The nearest station is Kami-Yonai on the JR Yamada Line, about a 2-minute walk from the plant. By bus, take the Iwate Kenkotsu Eki-Sakuradai Danchi line to the Sakuradai Danchi Higashiguchi stop, then walk about 15 minutes.