Mase Weir: Eastern Japan's Oldest Agricultural Concrete Gravity Dam
Tucked into a quiet mountain valley in Honjo City, Saitama Prefecture, the Mase Weir (間瀬堰堤, Mase Entei) stands as one of the most remarkable feats of pre-war civil engineering in eastern Japan. Completed in 1937 after eight years of construction, this concrete gravity dam was built to prevent flooding and secure irrigation water along the upper reaches of the Mase River, a tributary of the Tone River system. Today, it holds the distinction of being the oldest surviving agricultural concrete gravity dam in eastern Japan, and both the weir and its management bridge have been designated as Registered Tangible Cultural Properties of Japan since 2000.
The dam created Lake Mase (間瀬湖), a serene reservoir nestled within the Saitama Prefectural Jobu Natural Park. Together, the dam and lake form a landscape of exceptional beauty that has earned recognition as one of the "New 100 Scenic Views of Japan," a selection of "Furusato Saitama 100," and one of the "Tone River 100 Scenic Views." In 2010, Lake Mase was also selected as one of the Ministry of Agriculture, Forestry and Fisheries' "100 Best Ponds of Japan."
History and Construction
The Mase Weir was conceived as the centerpiece of the Kodama Irrigation System, designed to provide reliable agricultural water to the farming communities of what are now Honjo City, Misato Town, and parts of Fukaya City. Planning began in the Taisho era, and full-scale construction was carried out from 1930 (Showa 5) to 1937 (Showa 12), spanning a total of eight years.
The dam was designed by the engineer Takahori Ikuzo, whose work reflected the advanced civil engineering techniques of the early Showa period. The structure is a concrete gravity dam standing 27.5 meters tall with a crest length of 126 meters and a total storage capacity of 530,000 cubic meters. The upstream face features intake openings at three levels — upper, middle, and lower — allowing flexible water withdrawal depending on the reservoir level. The spillway employs a central overflow, non-regulated design. A distinctive two-story wooden intake tower stands on the upstream side of the dam, adding a striking architectural element to the structure.
The following year, in 1938, the Mase Weir Management Bridge was constructed downstream of the dam body. This reinforced concrete girder bridge spans 9.5 meters with a width of 2.0 meters, featuring elegant arched openings beneath its four piers. Grooves for stop-logs are set into the piers, reflecting the practical engineering requirements of water management. Both structures were registered as Tangible Cultural Properties on October 18, 2000.
Why It Was Designated a Cultural Property
The Mase Weir received its designation as a Registered Tangible Cultural Property of Japan in recognition of several important factors. As the oldest surviving agricultural concrete gravity dam in eastern Japan, it serves as an invaluable witness to the development of modern irrigation infrastructure during the early Showa period. The dam's design by Takahori Ikuzo showcases sophisticated engineering solutions, including the multi-level intake system and the central overflow spillway. The use of imitation-wood decorative parapets along the dam crest adds an unusual aesthetic dimension to what is fundamentally a utilitarian structure.
The management bridge downstream is likewise valued for its well-preserved early Showa-era reinforced concrete construction, with its distinctive arched pier openings representing a design sensibility that balanced structural function with visual appeal. Together, the dam and bridge form a cohesive ensemble of pre-war civil engineering heritage that continues to serve its original irrigation purpose nearly nine decades after completion.
The Japan Society of Civil Engineers has also recognized the Mase Weir in its "2000 Selected Important Civil Engineering Structures of Modern Japan," further affirming its significance in the history of Japanese infrastructure development.
Highlights and Attractions
The Dam Structure
Viewed from the downstream side, the Mase Weir reveals an impressive wall of concrete rising 27.5 meters above the valley floor. The evenly spaced openings along the top create a castle-wall appearance that has delighted visitors and dam enthusiasts alike. Walking across the dam crest offers panoramic views of Lake Mase on one side and the forested valley below on the other.
Lake Mase and Seasonal Beauty
The reservoir created by the dam is known as Lake Mase, and it is famous for its spectacular seasonal scenery. In spring, hundreds of cherry blossom trees lining the lakeshore burst into bloom, their reflections shimmering across the still water. Autumn brings vivid foliage of red maples and golden leaves that create equally stunning reflections. The area is also known for peach blossoms and daffodils at the nearby "Hanamomo to Suisen no Oka" (Peach Blossom and Daffodil Hill).
Fishing at Lake Mase
Lake Mase is one of the most popular fishing destinations in the region, renowned for its excellent crucian carp (herabuna) and Japanese smelt (wakasagi) fishing. The lake attracts anglers year-round, and rental boats are available from several operators around the lakeside.
Walking and Nature
The surrounding area lies within the Saitama Prefectural Jobu Natural Park, offering peaceful walking paths through lush forests. Recent improvements have enhanced the promenades near the dam, including rest pavilions (azumaya) and waterside paths that make for pleasant strolls in any season.
Nearby Attractions
The Kodaira district of Honjo City surrounding the Mase Weir is rich in cultural and natural attractions that can be combined into a rewarding day trip.
- Joshinin Hyakutai Kannon-do (Sazae-do) — One of Japan's three great "turban shell halls," this unique spiral-structured temple was built in 1783 to memorialize victims of the eruption of Mount Asama. Visitors can walk the helical corridor through three levels housing 100 Kannon statues. Admission: 300 yen.
- Takamado no Sato (High Window Village) — A picturesque hamlet of traditional sericulture farmhouses with distinctive raised ventilation windows (takamado), preserving the architectural heritage of Saitama's silk-producing past.
- Nihon Shrine (Japan Shrine) — Believed to be the only shrine in Japan bearing the name "Nihon" (Japan), this atmospheric forest shrine is reached via a mystical stone stairway.
- Honjo Tourism Agriculture Center — A visitor center offering local soba noodle making experiences, bicycle rentals (1,000 yen/day), and seasonal agricultural activities.
- Hanawa Hokiichi Memorial Museum — Dedicated to the blind Edo-period scholar Hanawa Hokiichi, this museum showcases his literary achievements and personal artifacts.
- Nagatoro — The scenic town of Nagatoro, famous for its river boat tours and rock formations, is accessible via the Prefectural Route 287 (Nagatoro-Kodama Line) that passes alongside Lake Mase.
Seasonal Guide
Spring (Late March – April): Cherry blossoms around Lake Mase create one of northern Saitama's most beautiful hanami settings. The Honjo City government organizes guided walking tours of the dam and surrounding area during cherry blossom season. Peach blossoms and daffodils bloom nearby.
Summer (June – August): Lush greenery surrounds the lake, and the forested park offers cool shade. Hydrangea, mountain lilies, and sunflowers add color to the landscape.
Autumn (October – November): Spectacular autumn foliage is reflected in the lake surface, creating vivid mirror-like scenes. Guided autumn walking tours featuring the dam are also available. Buckwheat flowers bloom in nearby fields.
Winter (December – February): The quietest season offers the most solitary views of the dam. Wintersweet (robai) blossoms add fragrance to the cold air. Wakasagi smelt fishing season attracts ice-fishing enthusiasts.
Access and Practical Information
By Train and Bus: Take the JR Hachiko Line to Kodama Station. From there, board the "Hanipon-go" community bus (reservation required: 0495-21-7797, weekdays only) to the "Mase-ko Entei" stop. Alternatively, take a taxi from Kodama Station (approximately 10 minutes).
By Car: From the Kan-Etsu Expressway, exit at Honjo-Kodama IC. Follow National Route 462, then Prefectural Route 287 (Nagatoro-Kodama Line) for approximately 20 minutes to reach Lake Mase.
Parking: Free parking is available near the downstream side of the dam and at the Honjo Tourism Agriculture Center.
Admission: The dam and lake area are free to visit at all times.
Contact: Honjo City Tourism Association, Kodama Branch — Tel: 0495-72-1334 (weekdays), 0495-72-1331 (weekends and holidays).
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Q＆A
- What makes the Mase Weir historically significant?
- The Mase Weir is the oldest surviving agricultural concrete gravity dam in eastern Japan. Completed in 1937, it represents an important milestone in the modernization of Japan's irrigation infrastructure. Both the dam and its management bridge have been designated as Registered Tangible Cultural Properties since 2000, and the dam is included in the Japan Society of Civil Engineers' list of 2000 important modern civil engineering structures.
- When is the best time to visit the Mase Weir and Lake Mase?
- Spring (late March to mid-April) is ideal for cherry blossoms reflected in the lake, while autumn (late October to November) offers stunning foliage views. The dam and surrounding park can be enjoyed year-round, with each season offering its own charm, from lush summer greenery to peaceful winter solitude.
- Is the Mase Weir accessible by public transportation?
- Yes, though it requires some planning. Take the JR Hachiko Line to Kodama Station, then use the "Hanipon-go" community bus (advance reservation required on weekdays) to the "Mase-ko Entei" stop, or take a taxi from the station (about 10 minutes). By car, it is approximately 20 minutes from the Honjo-Kodama IC on the Kan-Etsu Expressway.
- Can I fish at Lake Mase?
- Yes, Lake Mase is one of the most popular fishing spots in the region, especially for crucian carp (herabuna) and Japanese smelt (wakasagi). Rental boats are available from several operators around the lake, including Tanaka-en (0495-72-2369), Tsuribune Ichiriki (0495-72-1314), and Sansuiso (0495-72-2570).
- Is the Mase Weir still in active use?
- Yes. Nearly nine decades after its construction, the Mase Weir continues to fulfill its original purpose. Agricultural water generated at the dam flows through the Kodama Irrigation Channel to supply farmland in Honjo City and Misato Town. The dam is maintained by the Mikosawa Irrigation Land Improvement District, which conducts regular upkeep including mowing and cleaning.
Basic Information
| Name | Mase Weir (間瀬堰堤 / Mase Entei), also known as Mase Dam |
|---|---|
| Cultural Property Designation | Registered Tangible Cultural Property (Building) — Registered October 18, 2000 |
| Designer | Takahori Ikuzo (高堀育三) |
| Construction Period | 1930 (Showa 5) – 1937 (Showa 12) |
| Structure | Concrete gravity dam with intake tower and embankment levee |
| Dimensions | Height: 27.5 m, Crest length: 126 m, Total storage: 530,000 m³ |
| Management Bridge | Reinforced concrete girder bridge, 9.5 m long, 2.0 m wide (built 1938) |
| Owner | Mikosawa Irrigation Land Improvement District (美児沢用水土地改良区) |
| Location | Kodamacho Kodaira, Honjo City, Saitama Prefecture 367-0214 |
| Access | Approx. 20 min by car from Honjo-Kodama IC (Kan-Etsu Expressway); approx. 10 min by taxi from JR Kodama Station |
| Admission | Free |
| Other Designations | New 100 Scenic Views of Japan, Furusato Saitama 100, Tone River 100 Scenic Views, 100 Best Ponds of Japan, JSCE 2000 Selected Important Civil Engineering Structures |
References
- 間瀬堰堤 — 文化遺産オンライン
- https://bunka.nii.ac.jp/heritages/detail/167212
- 間瀬堰堤管理橋 — 文化遺産オンライン
- https://bunka.nii.ac.jp/heritages/detail/181813
- 農林水産省 農業用施設の文化財 — 間瀬堰堤
- https://www.maff.go.jp/j/nousin/sekkei/museum/m_bunka/yuukei1/
- 間瀬湖 — 本庄市観光協会
- https://www.honjo-kanko.jp/sightseeing/maze-ko.html
- Mase Lake — CHOCOTABI SAITAMA
- https://en.chocotabi-saitama.com/spot/44965
- Lake Ma-ze — Saitama Tourism Support Desk
- https://saitama-supportdesk.com/spots/post-21889/
- 間瀬ダム — Wikipedia
- https://ja.wikipedia.org/wiki/%E9%96%93%E7%80%AC%E3%83%80%E3%83%A0
- 間瀬ダム — ダム便覧
- http://damnet.or.jp/cgi-bin/binranA/All.cgi?db4=0633
- 間瀬ダム・間瀬湖 — たびまぐ
- https://tabi-mag.jp/sa0178/
- 埼玉県北部地域振興センター本庄事務所 — 桜の間瀬堰堤
- https://www.pref.saitama.lg.jp/b0111/tiikimeguri/kodamasatoyamagenkitaivol3.html
Last verified: 2026.03.26