Where Yokohama's Water First Slowed Down
The Former Aoyama Settling Basin of the Yokohama Waterworks Bureau's Aoyama Water Source Office (横浜市水道局青山水源事務所旧青山沈殿池, also written Kyū Aoyama Chindenchi) is a concrete sedimentation basin at Aoyama, Midori Ward, Sagamihara City, Kanagawa Prefecture, dated to 1897 and entered on Japan's national register of Tangible Cultural Properties on 9 October 1998.
A settling basin does one unglamorous job. River water arrives carrying suspended particles. Hold it still for long enough and gravity pulls the heavier material to the floor, so the water drawn off the top is measurably cleaner than the water that came in. No chemicals, no machinery, no energy input. Only patience and surface area.
At Aoyama the surface area is a square of 34.2 metres on each side, built in concrete and divided into two compartments, with the sloping inner faces of the walls faced in river cobbles. It stands directly above the Doshi River, a short distance from the intake structure that fed it.
The pairing was deliberate. Yokohama had begun supplying water in 1887 from an intake at Mii, where the Doshi joins the Sagami River, in a system designed under the British engineer Henry Spencer Palmer and recognised as Japan's first modern waterworks. By 1890 the city's population had passed 120,000 against a design figure of 70,000 served, and in 1897 the intake point moved roughly four kilometres upstream to Aoyama, high enough for water to enter by gravity rather than pumping. The settling basin belongs to the works that followed.
The First Expansion, and Water That Went Two Ways
Yokohama's First Expansion Works ran from 1898 to 1901 and produced the Kawai Water Treatment Plant in what is now Asahi Ward. The Agency for Cultural Affairs records the settling basin as a remnant of that programme, completed in 1901, while listing its construction year as 1897. Sagamihara City dates the start of the expansion to 1898. The dates sit slightly apart, which is common for works built in stages across several seasons.
What the basin did once it was running is clearly documented. Water settled at Aoyama went to two destinations: the newly built Kawai plant, and the existing Nogeyama plant that had served the city since 1887. One basin, two branches, an old system and a new one drawing from the same square of still water.
Kawai has outlived the arrangement in unexpected fashion. Completed in 1901 with a capacity of about 1,000 cubic metres a day, it remains the oldest of Yokohama's operating treatment plants, and after a 2009 rebuild under a private finance scheme it reopened in April 2014 using ceramic membrane filtration at 172,800 cubic metres a day. The plant still runs on the same principle that recommended Aoyama in the first place: the height difference between the Doshi River and the site provides the pressure, without pumping.
The basin itself was reduced by roughly half during the Second Expansion Works, the programme that completed the Nishiya Water Treatment Plant in 1915. What survives is the older, larger footprint's remaining portion, which is why the recorded dimensions and the description of "roughly thirty metres square" in the official commentary do not quite align.
Registered for the Landscape It Makes
Japan's registration system for cultural properties (登録, tōroku) was created in 1996 as a lighter alternative to designation (指定, shitei), aimed at modern structures that remain in use. Registration obliges the owner to notify the authorities before major alteration, and little more.
The Former Aoyama Settling Basin carries registration number 14-0024, registered on 9 October 1998 and announced on 26 October. The criterion recorded against it is different from the one applied to the neighbouring intake. Where the intake was registered as something not easily reproduced, the basin was registered as a structure contributing to the historic landscape of the nation (国土の歴史的景観に寄与しているもの).
That phrasing rewards a moment's thought. It is not a claim about rarity or technical ingenuity. It is a claim about how a place looks and how it reads. A large geometric water surface set into a wooded river valley, its banks lined with cobbles gathered from the river below, produces a scene that is neither natural nor industrial in any ordinary sense. Photographers who have worked the site describe the difficulty of taking it in whole; the trees and the fall of the ground keep breaking up the view. The compound has been used as a filming location.
In 1985 the Aoyama facilities were included in the Japan Water Works Newspaper's selection of one hundred notable works of modern Japanese water supply, thirteen years before the heritage registration.
Practical Notes for Visitors
The Aoyama Water Source Office is a working drinking water facility. Its perimeter is fenced with anti-climb barriers and the grounds are closed to unbooked visitors, so there is no wandering in for a look at the basin.
Booked tours are possible. The Yokohama Waterworks Bureau accepts facility visits at three sites, Aoyama among them, on weekdays excluding national holidays and the year-end break. Sessions begin at 9:30 in the morning and 13:30 in the afternoon, run about two hours, and take up to forty people. Applications go to the Waterworks Bureau Customer Service Centre by telephone (045-847-6262) or fax (045-848-4281), with the earliest date falling five business days after receipt; the fiscal 2026 booking window opened on 1 February 2026. The process runs in Japanese.
Set expectations accordingly. The programme is built around primary school environmental education and general public outreach on how drinking water is treated, and the Bureau specifically asks visitors to avoid using it for corporate inspection or technical study. Access to the registered basin is not part of any advertised itinerary, and photography follows staff direction.
Outside the fence, the road along the opposite bank of the Doshi River offers the best available view, though even in winter the full extent of the basin is hard to see at once. Reaching Aoyama: from JR Hashimoto Station, a Kanachu bus toward Mikage (三ヶ木) reaches the terminus in around an hour, leaving a thirty-minute walk; the service via Shin-Ogurabashi stops at Aoyama, ten minutes on foot. Parking is not provided and public transport is requested.
- 📍 Plan Your Visit to Kanagawa
- Find tours, activities & transport passes near Former Aoyama Settling Basin at the Aoyama Water Source Office, Yokohama City Waterworks Bureau
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Q&A
- Can I visit the Former Aoyama Settling Basin, and what does it cost?
- The basin lies inside an operating Yokohama Waterworks Bureau facility that is fenced and closed to unbooked visitors. Free guided tours of the Aoyama Water Source Office can be arranged in advance through the Waterworks Bureau Customer Service Centre, on weekdays from 9:30 or 13:30 for roughly two hours, for groups of up to forty. Applications are handled in Japanese by phone or fax.
- How large is the Former Aoyama Settling Basin?
- The Agency for Cultural Affairs records it as a concrete structure 34.2 metres long and 34.2 metres wide, divided into two compartments with cobble-faced sloping inner walls. Its official commentary describes an original scale of roughly thirty metres square, reduced by about half during the Second Expansion Works.
- Why was the Former Aoyama Settling Basin registered rather than designated an Important Cultural Property?
- Registration is a separate 1996 framework for modern structures still in service, requiring notification before major alteration instead of the strict controls attached to designation. The basin was registered under the criterion of contributing to the historic landscape of the nation, a different ground from the one recorded for the intake beside it.
- Where did water from the Former Aoyama Settling Basin go?
- It was sent to two treatment plants: the Kawai plant, newly built during the First Expansion Works and completed in 1901, and the Nogeyama plant that had operated since the waterworks opened in 1887. Kawai is still running today, rebuilt in 2014 with ceramic membrane filtration.
- What is the difference between the Former Aoyama Settling Basin and the Former Aoyama Intake?
- They are two separately registered structures on the same site, both dating to 1897 and both registered on 9 October 1998. The intake admits water from the Doshi River through cobble-packed iron grillework and Glasgow-made iron pipes; the settling basin holds that water still so suspended matter can drop out before it travels on toward Yokohama.
Basic Information
| Name | Former Aoyama Settling Basin, Aoyama Water Source Office, Yokohama Waterworks Bureau (横浜市水道局青山水源事務所旧青山沈殿池) |
|---|---|
| Location | 3482 Aoyama, Midori Ward, Sagamihara City, Kanagawa Prefecture |
| Designation | Registered Tangible Cultural Property (Structure); registration number 14-0024 |
| Year Designated | Registered 9 October 1998; publicly announced 26 October 1998 |
| Quantity | 1 unit |
| Dimensions | Concrete construction, 34.2 m long by 34.2 m wide; two compartments with cobble-faced slopes |
| Owner | Yokohama Waterworks Business Manager |
| Public Access | Inside an operating water supply facility; closed to unbooked visitors. Guided tours of the Aoyama Water Source Office by advance application, weekdays only |
References
- National Cultural Properties Database, Agency for Cultural Affairs — Former Aoyama Settling Basin
- https://kunishitei.bunka.go.jp/heritage/detail/101/00000822
- Cultural Heritage Online — Former Aoyama Settling Basin
- https://online.bunka.go.jp/heritages/detail/166132
- Sagamihara City — Registered National Cultural Properties
- https://www.city.sagamihara.kanagawa.jp/kankou/bunka/1022295/bunkazai/list/kuni_toroku/1010116.html
- Yokohama Waterworks Bureau — Kawai Water Treatment Plant Overview
- https://www.city.yokohama.lg.jp/kurashi/sumai-kurashi/suido-gesui/suido/suishitsu/josuijo/kawai/gaiyou.html
- Yokohama Waterworks Bureau — Facility Tours
- https://www.city.yokohama.lg.jp/kurashi/sumai-kurashi/suido-gesui/suido/torikumi/PR/kengaku.html
Last verified: 2026.08.13