Urban Waterfront Regeneration: How Wildlife and Cool Air Return to Rivers

Why did urban waterfronts become polluted?

Urban rivers and canals once used for water transport and daily life were forced, during the period of rapid economic growth, to absorb domestic and industrial wastewater, and quickly became polluted. As urban populations surged, sewer infrastructure failed to keep pace, and it was not unusual for detergents, oil, and garbage to flow directly into waterways. In particular, many Japanese cities adopted "combined sewer systems," which carry rainwater and wastewater through the same pipes, creating a structural problem in which untreated sewage overflows into rivers during heavy rain.

The challenge of combined sewer systems

According to the Tokyo Metropolitan Government Bureau of Sewerage, combined sewer systems are designed so that during heavy rain, a mixture of wastewater and rainwater is discharged into rivers and the sea via outlets and pumping stations along waterways, in order to protect urban areas from flooding. This is one of the main reasons rivers suddenly become polluted during rainfall. The first substantial rain after a dry spell is especially damaging, since it washes away accumulated grime from road surfaces and sewer pipes all at once.

Tokyo is pressing ahead with facilities to curb the outflow of debris from rainwater outlets and pumping stations, expanded sewer pipes to send more wastewater to water reclamation centers during rain, and storage facilities to temporarily hold the especially dirty wastewater from the start of a storm. Wastewater held in these storage facilities is sent to a water reclamation center after the rain stops and treated biologically as usual. As a result of these measures, by the end of fiscal 2023 the discharge water quality had improved to a level equivalent to that of separate sewer systems, as stipulated by the Sewerage Act Enforcement Order.

The era when expressways covered the waterfront

Many of the Shuto (Metropolitan) Expressway routes built for the 1964 Tokyo Olympics were constructed above rivers and canals, where land acquisition was easier. The Nihonbashi River is one such example, and the sight of an elevated highway covering the river has long been seen as a symbolic problem for "Tokyo, the city of water." Similarly, mid-sized urban rivers such as the Shibuya River were successively covered over due to declining water volume and sanitation concerns brought on by urbanization, disappearing from the surface as culverts.

Yanagawa's canals and the landfill plan that shook them

The city of Yanagawa in Fukuoka Prefecture has a network of canals, called "horiwari," stretching some 930 km. These canals, which had supported the castle town's domestic water supply, irrigation, and water transport since the Edo period, became severely polluted during the rapid growth era by synthetic detergents, oil, and garbage. At the time, officials within the administration favored a plan to fill in the canals and convert them into sewer channels — a choice many cities across Japan made during the same period.

Many waterways disappeared across Japan during this era. Culverting and landfilling were carried out as the "rational choice" to secure land for road widening and sewer construction, and as a result, the very presence of waterfronts faded from the memory of cities. What makes Yanagawa's case special is that a plan already on the verge of landfilling was reversed through the persistent persuasion of a single official.

The canals' value as an "invisible asset"

At the time, Yanagawa's canals were seen merely as old waterways, and replacing them with the "modern infrastructure" of sewers was viewed as a sign of progress. In reality, however, the canals simultaneously performed multiple functions: supplying irrigation water, diluting domestic wastewater, maintaining groundwater levels, and serving as a tourism resource. Re-evaluating this value as an "invisible asset" was the deciding factor in shifting the policy from landfilling to dredging.

This pattern is not unique to Yanagawa. In many cities, rivers and waterways were evaluated only for the single function of "flood control" or "drainage," while composite functions such as ecosystem services, cultural value, and groundwater recharge tended to be overlooked. In recent urban planning, frameworks that assess these multifaceted values of waterfronts in advance, before judging whether development should proceed, are gradually spreading.

Technologies and mechanisms for restoring water quality

The first step in waterfront regeneration is improving water quality. Approaches vary by city, but broadly fall into three categories: advanced sewage treatment, water level and flow management, and dredging and maintenance of the waterway itself.

Advanced sewage treatment (the Osaka Dotonbori River example)

To improve water quality in the Dotonbori and Higashiyokobori rivers, the city of Osaka introduced ultra-advanced treatment using the membrane bioreactor (MBR) process at the Nakahama Sewage Treatment Plant. By combining activated sludge treatment with membrane filtration, this method returns extremely clean treated water to the rivers, with high transparency and almost no detectable E. coli. Compared with conventional treatment methods, the equipment becomes more compact while treated water quality improves dramatically.

Water level and quality management with floodgates

The Dotonbori and Higashiyokobori rivers are sectioned off at both ends by water-level-adjustable floodgates, which prevent inflow of polluted water from upstream while stabilizing water levels. It is precisely this water level management that made it possible to build a promenade close to the water's surface. A stable water level is also an important condition for suppressing abnormal algae growth and maintaining greened embankments over the long term.

Yanagawa's canals, restored through dredging and citizen participation

In 1977, an official who had just been appointed head of the urban drainage section in Yanagawa pressed the mayor at the time not to fill in the canals, urging that the plan to convert them into sewer channels be halted. He demonstrated that Yanagawa's ground is soft "gata" (tidal-flat mud), and that if the water infiltration provided by the canals were lost, it could cause serious land subsidence — grounds that helped reverse the plan.

The "river purification plan" formulated in its place had three pillars: ① restore the waterways and secure flowing water through dredging, ② tighten drainage regulations to curb the inflow of sewage, and ③ build a maintenance system that revived citizen-participation cleanups. As a result, cooperation spread locally — illegal structures built over the waterways were voluntarily removed — and the goal is said to have been achieved at roughly one-fifth the cost of the original sewer-conversion plan.

Cost comparison: "filling in" versus "restoring"

Another lesson from Yanagawa's example is that waterfront regeneration is not necessarily a high-cost choice. A sewer-conversion plan would require laying new pipes and large-scale landfilling work, whereas restoration through dredging makes use of the existing waterway, dramatically reducing initial investment. In addition, making maintenance a citizen-participation effort reduces the management costs the administration would otherwise bear alone. Beyond the environmental benefits, it also made more financial sense to "repair and make use of what exists" than to "tear down and rebuild" — a rationale that later helped turn Yanagawa's canals into a tourism resource.

The cooling effect of waterfront regeneration

Urban waterfront regeneration brings measurable benefits not only to water quality but also to urban temperature. The world's leading demonstration of this is the restoration project of the Cheonggyecheon stream in Seoul, South Korea.

Cheonggyecheon's restoration and the "wind corridor" effect

The Cheonggye elevated highway, which had run east-west through central Seoul, was demolished between July 2003 and September 2005, and the culverted Cheonggyecheon stream was restored above ground along a length of about 5.8 km. Japan's National Institute for Environmental Studies conducted joint meteorological observations with the Korea Meteorological Administration's National Institute of Meteorological Sciences to verify the cooling effect on the urban block created by the "wind corridor" formed after restoration.

The results confirmed that a westerly wind flowing along the streambed dispersed into the north-south streets that intersect the river, producing a temperature drop of 0.3–0.4°C for every 1 m/s of wind speed. It was also confirmed that the amount of sensible heat transferred from the ground surface to the atmosphere dropped sharply, from 500 W/m² to 200 W/m². This case is regarded as the world's first demonstration that improving urban form can be effective in mitigating the urban heat island effect.

Common ground with Japan's urban green spaces and waterfronts

As explained in Urban Greenery and the Heat Island Effect, street trees and parks also lower temperatures, but waterfronts are distinctive in having a cooling source — the water surface — that green spaces lack. A linear waterfront that secures a wind corridor can contribute to heat island countermeasures in a way that differs from areal park greenery. In Tokyo, the Shibuya River is fed by a "clean stream revival project" that diverts advanced-treated sewage water into the channel, since securing the water volume itself is a precondition for both the cooling effect and the waterfront landscape.

Securing water volume through water diversion

In many urbanized areas, mid-sized rivers have seen declining spring water and groundwater seepage, leaving them with too little flowing water to begin with. In the Shibuya and Nomi rivers, Tokyo's "Jonan Three Rivers Clean Stream Revival Project" diverts advanced-treated water from sewage treatment plants to secure both water volume and quality. Without water, neither the cooling effect from evaporation nor ecosystem recovery can be expected, making water diversion one of the foundational measures for waterfront regeneration.

Building heat countermeasures into urban planning through waterfront design

What Cheonggyecheon's demonstration results showed is that it is not the waterfront alone, but the entire "urban form" — including the surrounding street layout and building orientation — that determines the thermal environment. By deliberately placing streets perpendicular to the river so that wind can reach deep into the city, the cooling effect spreads even to areas far from the water. In Japanese urban development too, an increasing number of redevelopment projects simulate wind flow at an early planning stage and design the waterfront and street network together. The perspective of not merely "cleaning up the river" but "spreading the river's effect across the whole city" is likely to become more important in future waterfront regeneration.

Building waterfronts where wildlife returns

Once water quality improves, the next change to appear is in the wildlife itself. Ecosystem recovery in urban rivers proceeds through a combination of measures: water purification, stable water levels, greened embankments, and the restoration of aquatic plants.

Greening embankments and aquatic plants

Vertical concrete embankments deprive wildlife of places to climb onto the water's edge or hide. Recent waterfront regeneration projects have adopted the concept of "near-natural embankments," in which part of the embankment is given a gentle slope planted with aquatic plants, creating an environment usable by fish, aquatic insects, and birds.

• Securing hiding places for wildlife through sloped or natural-stone embankments
• Providing water purification and spawning grounds through aquatic plant vegetation
• Confirming recovery status through regular water quality monitoring
• Biotope-like functions from planting on greened former waterways and the tops of culverts

Waterfront ecosystems as a barometer of water quality

The types and numbers of fish and aquatic insects also serve as an indicator of a river's water quality. A shift from a state dominated by pollution-tolerant midges to one where oxygen-demanding stoneflies and caddisflies appear is a sign that water quality improvement is steadily progressing. As with satoyama and satoumi, urban waterfronts too can become places that flourish through human involvement.

Green promenades over culverts: another option

Not every urban river can be restored above ground. In Tokyo's Setagaya Ward, since the 1960s, "green promenades" have been developed making effective use of the space above mid-sized rivers that were culverted. Multiple promenades exist within the ward, including the Karasuyama River Promenade and the Kitazawa River Promenade, restoring former waterways as pedestrian spaces, evacuation routes, and linear green spaces. Even where the water surface itself does not return, the idea of leaving the former river course in the city as a "network of water and green" offers a third option, distinct from undergrounding or landfilling.

Linear spaces as ecological networks

Promenades and waterfronts also serve as an "ecological network," or corridor, connecting scattered parks and green spaces. Birds and insects find it difficult to move through built-up areas blocked by roads and buildings, but a continuous stretch of green space and water along a river course allows them to expand their range by following it. As with Setagaya's network of green promenades, even a culverted former waterway can retain some function as a movement corridor for wildlife if maintained as a linear planted belt. Regardless of whether water is present, leaving an "unbroken line" within the city is important from the standpoint of ecosystem conservation.

The local economic effects of waterfront vibrancy

Waterfront regeneration has ripple effects on the local economy as well as the environment. In a case study compiled by the Ministry of Land, Infrastructure, Transport and Tourism on the Dotonbori River, waterfront improvements are cited as having contributed to enhancing the city's appeal as an example of the "stock effect of infrastructure."

The effect of the Dotonbori River's "Tombori River Walk"

The city of Osaka built a promenade called the "Tombori River Walk" along the Dotonbori River's waterfront, realizing a waterfront space where people can walk close to the water's surface. Along both banks of the Minatomachi area, young people enjoying street dancing and strolling have become a fixture, and the strengthened north-south flow with the Horie area — known for its stylish shops and housing — has increased circulation with surrounding areas, reportedly raising the district's overall standing as a hub.

Integrating waterfront development with mixed-use redevelopment

Along the Nihonbashi River, five redevelopment projects covering a site area of about 6.7 hectares are planned alongside the undergrounding of the Shuto Expressway. The underground route is expected to be completed by fiscal 2035, with full completion — including removal of the current elevated section — expected by fiscal 2040. If realized, a vast waterfront space, including the river width, spanning about 100 m wide and roughly 1.2 km long, known as the "Nihonbashi River Walk," will be created. What is distinctive is that the waterfront itself is being positioned as the center of new urban functions, by advancing mixed-use development — commercial facilities, offices, and lodging — together with waterfront improvement as one integrated project.

On the other hand, the Nihonbashi River also faces water quality challenges, including high nitrogen and phosphorus concentrations that make it prone to phytoplankton and blue-green algae blooms, along with the ongoing problem of untreated water flowing in from the combined sewer system during heavy rain. Landscape restoration and water quality improvement need to be pursued together as two inseparable wheels.

Waterfronts as a tourism resource

Yanagawa's canals are now known as a tourist attraction where sightseeing boats ply the waters. Their value as a tourism resource only emerged once water quality recovered — the boat-ride experience itself could not have existed while the waterway remained polluted. Improving water quality is both an environmental measure and an economic foundation that supports a region's brand. Likewise, the promenade improvements along the Dotonbori River created a new appeal for Osaka as a place that is "enjoyable to walk along the water" for tourists.

Real estate value and the "waterfront premium"

In waterfront regeneration examples overseas, a "waterfront premium" is sometimes observed, in which surrounding land prices and office rents rise after improvement. Homes and offices facing a waterfront tend to be valued for their views and comfort, and in the Nihonbashi River redevelopment plan too, waterfront improvement is clearly positioned as a strategy for raising the value of surrounding real estate. That said, the waterfront premium is only realized once basic conditions — water quality, safety, and access — are in place, and it should be noted that improvement alone does not automatically raise land prices.

Urban waterfronts connected to the sea

Urban canals and rivers are, in most cases, directly connected to the sea. Improved water quality in urban areas directly affects the coastal marine environment downstream. Measures for urban rivers upstream are an indispensable piece of the puzzle in improving water quality in bays such as Osaka Bay and Tokyo Bay.

Links to the restoration of tidal flats and seagrass meadows

Improving water quality in urban rivers is closely linked to the conservation of tidal flats and the restoration of seagrass meadows at river mouths. Optimizing the inflow of nutrients such as nitrogen and phosphorus can also lead to the recovery of biodiversity living in tidal flats and seagrass beds. Conversely, excessive nutrient inflow from urban rivers and the outflow of untreated sewage can contribute to red tides and hypoxic water masses at river mouths, meaning that the effects of upstream measures reach coastal waters tens of kilometers away.

It is desirable for urban waterfront regeneration to be advanced not as an isolated river improvement project, but as a "watershed-scale" effort that looks all the way from upstream sewer infrastructure to the coastal marine environment at the river mouth. In Yanagawa's case too, improving the canals' water quality has ultimately been treated as a theme closely tied to the fishing environment of the Ariake Sea.

Thinking at the scale of the whole watershed

The water quality of a single river is determined by the accumulated conditions across the entire watershed — wastewater from upstream farmland, residential areas, and industrial zones, discharges along the way from combined sewer systems, and even the tidal ebb and flow near the river mouth. Improving only the city center has limited effect if a large volume of untreated wastewater is flowing in from upstream. Conversely, if water conservation and domestic wastewater measures advance upstream, the benefits cascade downstream to urban rivers and further out to the marine environment beyond. Viewing waterfront regeneration not as a "point" but as a "line," and further as a "plane" (the watershed), is what determines its long-term effectiveness.

Ripple effects on coastal fisheries

Nutrients and pollutants flowing in from urban rivers also directly affect fisheries at the river mouth. Excess nutrients can trigger red tides and damage coastal fisheries such as shellfish and nori (seaweed) aquaculture, while a moderate supply of nutrients also supports the productivity of the coastal ecosystem. When advancing urban waterfront regeneration, it is important not merely to "clean things up," but to work with fishery stakeholders and marine environment experts at the river mouth to find the right balance across the entire watershed.

Urban waterfront regeneration projects in Japan and the world

Urban waterfront regeneration is not a movement confined to Japan. Major cities around the world are also working to reclaim waterfronts lost during the era of motorization.

A common pattern for success

What these examples have in common is that they satisfy three elements simultaneously: ① infrastructure investment to secure water quality and volume, ② a mechanism to manage water level and flow, and ③ spatial design that lets people approach the water. Waterfront regeneration is unlikely to succeed with just one of these alone. In addition, as Yanagawa's example shows, projects with a mechanism that lets citizens become stakeholders in maintenance — not just administrative planning — tend to be more sustainable over the long term.

Differences in project scale and speed

Projects involving the removal or undergrounding of elevated highways, such as Cheonggyecheon and the Nihonbashi River, are large-scale undertakings requiring investment in the hundreds of billions of yen and timeframes of 10 to 40 years. Yanagawa's dredging project, on the other hand, achieved results relatively quickly and at low cost by making use of the existing waterway. Which approach is suitable depends on the scale of the city and the condition of the river. Some cities consider large-scale regeneration timed with the renewal of aging infrastructure, while others steadily protect water quality as an extension of routine maintenance — there is no single correct answer in waterfront regeneration.

Challenges facing waterfront regeneration

While waterfront regeneration has many success stories, challenges remain to be overcome. Understanding the actual constraints, rather than speaking only in ideals, is key to the success of the next regeneration project.

Fundamentally resolving combined sewer systems is not easy

Converting a combined sewer system into a separate system — one that completely divides wastewater and rainwater pipes — requires large-scale construction to lay new sewer pipes beneath roads. In already densely built-up city centers, this is often unrealistic in terms of both cost and construction time, so many cities take the approach of "improving while keeping the combined system," such as adding storage facilities or advanced treatment. This approach has its limits, and as record-breaking heavy rain becomes more frequent, it is difficult to reduce the risk of untreated sewage overflow to zero entirely.

Long project timelines and building consensus

As with the undergrounding of the expressway over the Nihonbashi River, it is not unusual for large-scale waterfront regeneration projects to take nearly 20 years from the start of construction to completion. Over such long timeframes, the original plan and social circumstances may change, requiring ongoing consensus-building with local residents and businesses. Even a relatively small-scale dredging project like Yanagawa's requires resident cooperation to strengthen drainage regulations — a waterfront cannot be regenerated by administrative decree alone.

Growing heavy-rain risk from climate change

Recent climate change is said to be increasing the frequency of short-duration intense rainfall. Even as improvements to combined sewer systems and storage facilities progress, if rainfall exceeds expectations, the risk of untreated sewage overflow rises again. Waterfront regeneration is not a one-time undertaking but a piece of social infrastructure that must be continuously updated in light of the effects of climate change.

How we can engage with our waterfronts

Urban waterfront regeneration tends to center on large-scale administrative projects, but there are no shortage of ways for residents and businesses to get involved. Just as Yanagawa's regeneration was supported by citizen-participation cleanup activities, small everyday actions add up to long-term water quality maintenance.

• Participating in river cleanups and waterfront wildlife observation events
• Reducing the burden on sewage treatment through water conservation and mindful domestic wastewater habits
• Participating in waterfront events, cafés, and other efforts to build vibrancy
• Participating in municipal river monitoring and public comment processes

Waterfront observation as environmental education

In areas where waterfront regeneration has progressed, waterfronts are also beginning to be used as sites for environmental education by schools and local organizations. Through observing aquatic life and conducting water quality surveys, learning how the familiar river or canal nearby became polluted and how it has recovered is essential for fostering a sense of ownership in the next generation. Just as Yanagawa's canals have become an educational resource for the local community as well as a tourism resource, the benefits of waterfront regeneration extend beyond the economy and the environment to human capital development as well.

Urban waterfronts are regenerated over time through the accumulation of small actions by government, businesses, and residents alike. As shown by Cheonggyecheon, the Dotonbori River, and Yanagawa — which avoided landfilling — many cities have already begun to prove that a waterfront once nearly lost can be reclaimed. The next time you walk along a river or canal in your city, take a moment to imagine where that water came from, and where it is flowing to.