Bringing Back the Beauty of the Outer Moat– Initiatives by the Outer Moat Waterfront Regeneration Council –

Have you heard of the sotobori, or, outer moat?
Many may be familiar with the outer moat as the waterfront scenery visible from train windows or while jogging nearby the Imperial Palace. This scene is part of everyday life for those who pass by regularly.

The outer moat refers to the exterior boundary moat that once surrounded Edo Castle. During the Edo period, (1604-1868), it was part of a larger network of waterways that connected the inner moats with Tokyo Bay (formerly called Edo Bay). Today, much of the moat has been filled in to transform the space into roads and parks, but some of the old castle’s stone walls and gates are still visible.

However, there are still a few sections where the original waterway remains: the three northern moats of Ushigome, Shimmitsuke, and Ichigaya—which run alongside the JR rail line from Iidabashi Station to Yotsuya Station—and Benkei Moat near Akasaka-mitsuke Station. Tecnnically, even the Kanda River, which merges with the moat system east of Iidabashi, could be considered part of the outer moat.

Today, however, water quality degradation in the remaining outer moat areas has become a significant concern.

The Birth of the Outer Moat Waterfront Regeneration Council

Areas where the outer moat still retains water

Waterfronts are meant to be relaxing, oasis-like spaces. However, the current state of the outer moat falls short of that ideal. Deteriorating water quality has made the moats murky and green, with foul odors emanating, thus the true charm of the moat is being lost.

In response to these issues, a group of business leaders whose offices overlook the waterways have come together to take action. Driven by a desire to improve the scenery and make it a more attractive place for visitors, Miyasaka, then-CEO of LINE Yahoo Corporation (formerly Yahoo Japan), took the initiative In May 2016, to create the Outer Moat Waterfront Regeneration Council.

The outer moat is not only a historic relic that once helped defend Edo Castle, it has also served as an important part of the city’s landscape from Edo times. Here we discuss what challenges must be addressed to rejeuvenate the waterway and the initiatives the Council has in mind to return it to its former glory.

A Major Issue: Blue-Green Algae and Foul Odors

When large amounts of phytoplankton—also known as blue-green algae—grow and cover the surface of the water, it leads to “algal blooms”. If left untreated, this algae decays and emits a strong, unpleasant odor. This is the problem currently affecting the outer moat.

The main cause is believed to be the stagnant water, which has effectively turned the moat into a reservoir, with little to no water movement. One contributing factor is that—since the development of the modern sewage system—portions of rainwater pass through the sewers and are discharged into the moat. This water often carries pollutants that accumulate in the moat.

The result is a body of water that looks as though it has been painted blue-green paint and gives off a foul aroma. The challenge is to purify this water to restore it’s scenic beauty.

Measure 1: Bringing in Clean Water from Outside

One potential solution is to introduce clean water from external sources into the outer moat. This involves drawing in water from nearby rivers to dilute the polluted water and reduce water retention time, thereby preventing the moat from turning stagnant.

To explore this possibility, in 2019, the Council partnered with Professor Suzuki of Hosei University to conduct computer-based simulations. These models helped determine the necessary water quality and flow volume needed to effectively shorten water retention time and improve overall water quality. The results were shared with the Tokyo Metropolitan Government.

As a result, the Tokyo Metropolitan Government is now advancing its own plans to draw water from the Arakawa River, and possibly from treated wastewater, into the outer moat, with the goal of completing the project in the 2030s.

The Outer Moat Waterfront Restoration Council will continue to work with Professor Suzuki to refine the simulation models and provide updated data and analysis to Tokyo authorities.

Measure 2: Preventing the Inflow of Polluted Water

Another key measure is to prevent polluted water—such as household and industrial wastewater normally sent to water treatment plants via the sewage system—from overflowing into the moat during heavy rains. When it rains heavily, stormwater can overwhelm the system and carry this untreated water directly into the outer moat.

To address this issue, simulations were again conducted with Professor Suzuki to analyze strategies such as constructing a protective waterway to redirect portions of rainwater and fashioning temporary holds to contain wastewater in order to prevent it from being released into the moat. The simulations showed that these methods could significantly improve water quality by both reducing polluted inflow during storms while increasing the amount of clean water entering the moat.

The analysis also revealed that combining this strategy with Measure 1—introducing clean water—is more effective than implementing either measure alone.

The Tokyo Metropolitan Government has already begun initiatives to temporarily store rainwater with high pollutant levels at the start of a rainfall event, and to release it to water treatment centers after the rain has stopped.

Measure 3: Purifying Water Quality Using Aquatic Plants

In addition to the previously mentioned measures, improving the quality of the existing water within the moats themselves is also essential. To achieve this, the Council turned its attention to the power of aquatic plants.

Through water quality surveys of the outer moat, it was found that the water in Benkei Moat had higher clarity and quality than the other moats. Several species of aquatic plants grow abundantly in Benkei Moat, including Myriophyllum spicatum, which research suggests has an allelopathic effect that helps suppress the formation of blue-green algae.

Allelopathy refers to the phenomenon in which chemicals released by one plant affect the growth or behavior of surrounding plants or microorganisms. Because this method of natural suppression can reduce the need for synthetic chemicals, it is increasingly being applied to agriculture purposes such as pest control and weed mitigation.

Given that moats like Ushigome, Shimmitsuke, and Ichigaya were historically connected to Benkei Moat, it is believed feasible to transplant aquatic plants from Benkei Moat to the others. This could help purify the waters across the broader moat waters.

Experiments to study the purification ability of aquatic plants are ongoing. The Council also plans to consider the feasibility of future transplantation and long-term maintenance, presenting its proposals and results to the Tokyo Metropolitan Government.

Protecting the History and Future of the Outer Moat

Over the outer moat’s long history, the waterway has remained a prominent feature of Tokyo’s urban landscape. We hope it becomes a place where local residents and visitors alike feel drawn to relax, connect, and enjoy the natural surroundings.

With this vision in mind, the Outer Moat Waterfront Restoration Council is committed to ongoing efforts to revitalize the area. While full restoration will take time, we will continue to share updates on our progress, including improvements in water quality. We will also promote the moat’s cultural and historical significance.

By raising awareness and fostering a sense of connection, we hope to inspire more people to take interest in the outer moat. This, in turn, will help pave the way for a brighter and more sustainable future for this treasured part of Tokyo.