Most people picture a street drain as the last place any living creature would want to be. Grey water, litter, the occasional smell — that's the mental image almost everywhere in the world. Then there's Shimabara, a small city on Japan's Kyushu island, where hundreds of koi fish swim openly through the drainage channels that run alongside the streets. Locals and visitors alike can lean over a low stone wall and watch full-grown carp glide past shopfronts and old wooden houses. It sounds like a rumour until you see the photos — and it turns out the story behind it is just as interesting as the sight itself.
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| Koi carp glide through a spotless drainage channel in Shimabara, Japan's "City of Water". |
Why Shimabara's Water Is Different
Shimabara sits at the base of Mount Unzen, an active volcano on the Ariake coast of southern Japan. In 1792, a violent earthquake and landslide connected to the volcano triggered a devastating tsunami that killed roughly 15,000 people — one of the deadliest disasters in Japanese history. Out of that tragedy came an unexpected side effect: the shifting geology opened up dozens of natural freshwater springs beneath the city. Today, Shimabara draws on more than 60 of these springs, and the constant flow of cold, mineral-filtered volcanic water gives the city one of the purest municipal water supplies in Japan.
A City Built Around Its Springs
Because the spring water never stops moving, it doesn't sit and stagnate the way water in an ordinary gutter would. That single detail — constant flow — is what makes fish-friendly canals possible. The city eventually earned the nickname "City of Water" for exactly this reason, and locals still take real pride in keeping the water that clean.
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| A bird's-eye view of Koi no Oyogu Machi, the street where hundreds of koi make their home. |
How the Koi Fish Got There
The koi were not always there. It wasn't until 1978 that Shimabara's neighbourhood association decided to formally introduce koi carp into a roughly 100-metre stretch of canal running through the older part of town, an area now nicknamed Koi no Oyogu Machi, or "Carp Swimming Street". The fish took to the environment immediately, and their population has grown steadily ever since, with some individuals now reported to reach 50 to 70 centimetres in length.
Why Koi Are the Perfect Indicator Species
Koi carp are notoriously picky about water quality. Unlike hardier fish, they need consistently clean, well-oxygenated, low-sediment water to survive long-term, which is precisely why their presence works as a kind of living proof of water purity. If the canal water in Shimabara ever became polluted or stagnant, the koi simply wouldn't be there. Residents understand this, which is part of why the community treats the canals as a shared responsibility rather than a municipal afterthought — homeowners along the route routinely sweep debris away and avoid letting anything drain into the water that doesn't belong there.
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| Some koi in Shimabara grow to 50–70 centimetres long, easily visible from street level. |
What It's Like to Visit
Walking through Shimabara today, the canals are visible right alongside the sidewalks, often just a low curb away from foot traffic. Locals stop to point out especially large or brightly patterned fish; some have even given individual koi informal names over the years. Visitors are generally asked not to feed the fish, since uncontrolled feeding can cloud the water and undo decades of careful upkeep. A short walk along Koi no Oyogu Machi takes maybe twenty minutes, but most people end up staying far longer than that, just watching the fish move against the current.
Beyond Shimabara
Shimabara isn't entirely alone in this regard. A handful of other Japanese towns — including Hida Furukawa, Gujo Hachiman, and Tsuwano — have their own versions of clean, fish-populated waterways, usually tied to similar access to mountain or spring water. But Shimabara's combination of scale, volcanic backstory, and sheer number of koi has made it the most recognised example and the one that keeps circulating on social media as proof that Japan takes water cleanliness to another level entirely.
Koi Fish in Japan's Drainage Canals: Inside Shimabara, the "City of Swimming Carp"
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| Small footbridges throughout Shimabara offer close-up views of the resident koi. |
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| The constant flow of spring water keeps the canals oxygenated and free of stagnation. |
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| No two koi in Shimabara look quite alike — patterns range from solid white to deep red and black. |
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| The water is clean enough that the canal floor stays visible even with dozens of fish swimming through it. |
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| Old minka-style houses still line the canal route through the historic part of town. |
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| A closer look at one of Shimabara's larger koi, distinguished by its bold colour markings. |
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| Visitors are welcome to watch the koi but are asked not to feed them to protect water quality. |
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| Koi no Oyogu Machi, or "Carp Swimming Street," is the most famous stretch of Shimabara's koi canals. |
Conclusion
Shimabara's koi-filled canals aren't a gimmick or a tourism stunt — they're the byproduct of a genuine natural disaster, a rare geological gift, and decades of quiet community discipline. What started as volcanic springs following a tragedy in 1792 became, through a deliberate decision in 1978, one of the more unusual and photogenic sights in Japan: hundreds of koi carp treating a city's storm drains as home. It's a small but striking reminder that infrastructure and nature can coexist and that some of the most memorable travel experiences come from looking down into a gutter rather than up at a landmark.
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References & Sources
Where in Japan can we find “Drainage Canal with Koi fish”?
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