The Impacts of Urbanization on Watersheds

Erin Brennan

Western Kentucky University

BIOL 475G Aquatic Ecology

28 July 2021

Urbanization of Watersheds

The urbanization of freshwater systems globally is rapidly accelerating and is of great ecological concern. This includes not only freshwater creeks, streams, and rivers, but wetlands as well. Many major cities around the world were built along rivers or wetlands that were filled in, built upon, and paved over during centuries of growth and development. This has led to urban flooding along with the pollution of freshwater systems in urban areas due to runoff and sewage overflow.

Impacts of Urban Development on Watersheds

The rapid urban development of the twentieth century focused very little on how urban watersheds should be treated (Khirfan et al. 2020). Wetlands were filled in and streams were subjected to manipulation, diversion, straightening, or confinement to channels and underground piping to accommodate sprawling urban centers (Khirfan et al. 2020). An example of this can be found in Louisville, Kentucky. Louisville was built on wetlands bordering the Ohio River. In the 1920s, the city of Louisville began to pave and convert much of the Beargrass Creek watershed into combined sewers to deal with issues of persistent flooding and the growing amount of raw sewage attributed to a rapidly expanding urban area. When the sewer system of Louisville was built, sewers were designed to flow directly into waterways and were later adapted to overflow into creeks and wetlands during storms (Combined Sewer Overflows. Kentucky Energy and Environment Cabinet). Sewage from homes and businesses, along with waste products from factories and slaughterhouses was dumped directly into Beargrass Creek or the Ohio River (Combined Sewer Overflows. Kentucky Energy and Environment Cabinet). In many areas, creeks were transformed into concrete sewer lines.

Before centralized wastewater treatment became widely available, combined sewers that collect sewage and stormwater runoff in a single pipe for transportation to a treatment plant were commonly used in cities around the world in the late nineteenth to early twentieth centuries (Combined Sewer Overflows. Kentucky Energy and Environment Cabinet). These combined sewer systems were continuously discharged into receiving waters to minimize the contact between people and sewage (Combined Sewer Overflows. Kentucky Energy and Environment Cabinet). While often successful in rerouting runoff and sewage away from people, combined sewers that overflow into urban waterways have resulted in heavy contamination.

Urbanization has also led to a rise in urban flooding, with an increase in annual surface runoff up to 413% and an increase of annual flood volumes up to 942% (Zhou et al. 2019). With wetlands drained and built upon, streams concreted in, and impermeable surfaces such as roads and parking lots paved over, excess rainwater has nowhere to go. Understanding the factors contributing to the rise in urban flooding is key to mitigating their impacts (Zhou et al. 2019).

Global Wetland Loss

Despite the crucial roles wetlands play in holding freshwater, preventing flooding, and providing critical habitat, up to 87% of global wetlands have been lost since 1700 (Ramsar Convention on Wetlands). For centuries, wetlands were seen as a source of disease, pestilence, and insects that held little value (Kim et al. 2010).

Globally, wetlands are lost at a rate three times that of forests (Ramsar Convention on Wetlands). Much of this loss has occurred because wetlands tend to be considered wasteland to be filled in or converted for other uses (Ramsar Convention on Wetlands). Some US states have lost more than 90% of their wetlands (Kim et al. 2010). Despite all of the disadvantages associated with developing wetlands, the drainage and development of wetlands is still continuing at a rapid pace, especially in urban and coastal areas (Lee 2006).

Causes of Wetland Loss

The main causes of the loss of wetlands are changes in land use, an increase in agriculture, water diversion through dams and canalization, and urbanization and infrastructure development, especially in river valleys and coastal areas (Ramsar Convention on Wetlands, Lee et al. 2006).

Urban and rural development account for over a third of wetland loss, with urbanization being a leading cause of coastal wetland loss (Lee et al. 2006). Like many watersheds across the country and around the world, the Beargrass Creek watershed in Louisville, Kentucky faces myriad challenges including urbanization, flooding, and pollution. Beargrass Creek encounters issues stemming from the urbanization of the areas surrounding the three branches of the creek that flow through much of the Louisville metropolitan area before joining the Ohio River.

Lake Karla in Greece is another example of a freshwater system that has experienced problems stemming from agriculture and urbanization. Lake Karla and the surrounding wetlands were almost completely drained in 1962 in an effort to increase agricultural production and protect surrounding areas from flooding (Zalidis et al. 2004). However, this drainage had unintended consequences. The subsequent loss of wetland functions resulted in an increase in environmental problems that led to social and economic issues (Zalidis et al. 2004).

Climate Change

Consequently, with the wetlands that once absorbed much of seasonal flood waters destroyed, flooding in areas adjacent to freshwater systems has continued to plague cities globally and is forecasted to worsen with climate change. Current climate change scenarios predict an increase of extreme rain events, which will lead to an increase in wastewater flooding, sewage overflow, and a decrease in water quality (Veronesi et al. 2014).

Seasonal and climate change-fueled flooding is exacerbated by the prevalence of impermeable surfaces across the urban landscape as well as by the transformation of urban creeks and streams into concrete drainage ditches or sewers. According to Ramachandra et al. (2012), frequent flooding in urban areas, including during normal rainfall events, is a consequence of the increase of impervious surfaces, high-density urban development, and loss of wetlands and vegetation. A study on the impacts of urbanization and the effects of climate change on a major city in Northern China found that annual urban flood volumes could increase by up to 200% due to climate change using the RCP 2.6 emissions scenario (Zhou et al. 2019). In Louisville, Kentucky, a city with a floodplain up to 5 miles wide, a catastrophic flooding event could impact over 200,000 people and cause up to $34 billion in property damages (WFPL).

Figure 1. Climate change brings major flood risks to Louisville, as shown in this flood simulation map developed by the Army Corp of Engineers and Louisville/Jefferson County Metropolitan Sewer District.

A recent study conducted in Fairfax, VA indicated that climate change simulations showed that annual runoff volume could increase by 6.5% (Alamdari et al. 2017). Not only will urbanization and climate change result in higher runoff volume, but suspended solids, nitrogen, and phosphorous levels will increase as well (Alamdari et al. 2017). As a result, water treatment practices must be adjusted to deal with increasing urbanization and the impacts of climate change to meet water quality standards (Alamdari et al. 2017).

Wetlands are particularly sensitive to changes in both the quality and quantity of water (Erwin 2009). For this reason, climate change will have a pronounced effect on wetlands (Erwin 2009). Further, due to the loss of wetlands and native vegetation, the rate of biodiversity loss will increase (Kim et al. 2010). This scenario is not limited to a single urban area, but is occurring globally - the total loss of wetland areas and biodiversity worldwide due to urbanization is still being calculated (Kim et al. 2010).

Biodiversity Loss

When wetlands are lost, we also lose wetland-dependent biodiversity. Freshwater species have declined 81% and coastal and marine species 36% due in part to this loss of wetland resources (Ramsar Convention on Wetlands). Urban development and the pollution that comes with it generally result in changes to the algal, invertebrate, and fish communities (Coles et al. 2012). The most consistent change in urban biological communities is the loss of sensitive invertebrate species and a shift to species that are more tolerant to environmental stressors (Coles et al. 2012). Native riparian vegetation is also lost due to urban development. This loss of native species has contributed to the establishment of more tolerant invasive plant and animal species (Coles et al. 2012).

Combined Sewage Overflows

In many urban areas around the world, stormwater and sewage systems were designed to overflow into creeks and wetlands. These combined sewers that transport sewage and stormwater runoff to a treatment plant were common before centralized wastewater treatment was available (Combined Sewer Overflows. Kentucky Energy and Environment Cabinet). Combined sewer overflows can greatly impact the health of receiving waterways in urban areas (Fortier & Mailhot 2015). Years of runoff and overflow from combined sewer systems have resulted in urban watersheds that are heavily contaminated with fecal bacteria and chemical pollutants. Concentrations of contaminants, such as insecticides, chloride, nitrogen, and polycyclic aromatic hydrocarbons (PAHs) generally increase with urban development (Coles et al. 2012; Olds et al. 2018). Sewage contamination levels are directly linked with the amount of urbanization and the density of impervious surfaces in watersheds (Olds et al. 2018).

Climate change and urban development will impact the effectiveness of combined sewer overflow systems (Fortier & Mailhot 2015). In fact, many urban drainage systems are already experiencing decreasing efficiency due to climate change and rapid urbanization (Yazdanfar & Sharma 2015). As urban drainage systems continue to decline in efficiency, sewer overflows and urban flooding will lead to increased pollution in urban waterways (Yazdanfar & Sharma 2015). Simulations conducted by Nilsen et al. (2011) have indicated that combined sewer overflow could increase as much as 83% in urban areas.

Solutions to the Urbanization of Watersheds

Long-term solutions to the problems plaguing urban watersheds exist. Clean-up efforts to remove junk, trash, and sewage are simply no longer enough. Restoration is needed to alleviate the problems associated with the urbanization of freshwater systems and wetlands globally.


Daylighting creeks, which is “the practice of removing buried streams from underground culverts and exposing them by bringing them to the surface of the earth” has been undertaken by some urban municipalities, including Louisville, KY (Khirfan et al. 2020; Louisville/Jefferson County Metropolitan Sewer District 2020). Stream daylighting can mitigate some of the impact of climate change by managing runoff during extreme rain events and can contribute to lessening the urban heat island effect (Khirfan et al. 2020). The removal of impermeable surfaces, also known as ‘depaving’ can also contribute to a reduction in urban flooding and the urban heat island effect (Khirfan et al. 2020).

Figure 2. A stream restoration project in Louisville, Kentucky is an example of the practices of daylighting in which buried streams are removed from underground culverts and depaving in which impermeable surfaces are removed.

Overflow Basins

One solution to the problem of stormwater runoff and sewage overflow is the building of overflow basins in areas that most frequently have overflow problems during heavy rain events. These basins catch runoff and sewage overflow so that it can be treated rather than being funneled directly into urban waterways. The construction of combined sewer overflow and wastewater basins to prevent water contamination during storms has begun in areas across Louisville, Kentucky particularly prone to flooding and sewage overflows (Louisville/Jefferson County Metropolitan Sewer District 2020). Despite prior efforts to clean up the Beargrass Creek watershed, it has remained heavily polluted due to runoff and sewage overflow. The addition of overflow basins is projected to reduce pollution levels in the three forks of the creek that flow into the Ohio River (Louisville/Jefferson County Metropolitan Sewer District 2020).

Figure 3. Louisville/Jefferson County Metropolitan Sewer District construction of Portland Combined Sewer Overflow Basin to prevent overflow of sewage into local waterways.

Wetland Restoration

Not only must existing wetlands be protected, but wetland restoration is critical to the future of these vital ecosystems and will mitigate some of the impacts of climate change (Kettenring & Tarsa 2020). According to the Ramsar Convention on Wetlands, the main goal of any restoration project “is to create resilient and sustainable ecosystems, as measured on a human timescale, in order to improve the ecological character and enhance the socioeconomic role that the wetland plays in the watershed.” (Zalidis et al. 2004). New tools, such as the Wetland Impact Assessment, can be used for wetland mitigation and restoration projects to minimize damage, restore or enhance wetlands, and even create new wetland environments (Kim et al. 2010).

Wetland restoration techniques used will depend upon the particular habitat. Floodplains, mangroves, salt marshes, peatlands, freshwater marshes, and forests are distinct environments that require different management and restoration techniques (Erwin 2009). It can be particularly challenging to evaluate the impacts of climate change on urban watersheds and wetlands, but the impacts of climate change must be taken into consideration when restoring a wetland environment, particularly near an urban area (Alamdari et al. 2017). Major wetlands around the world are experiencing different effects of climate change, from the Sundarban Mangrove in India, to the Mekong River delta in Vietnam, to the Mississippi River delta in Louisiana - each of these unique ecosystems has its own set of restoration challenges (Erwin 2009).

While often overlooked in restoration projects, an important first step for restoring any wetland is restoring native vegetation (Kettenring and Tarsa 2020). Wetland restoration projects to aid in flood prevention have begun in former wetland areas of the Beargrass Creek watershed in Louisville, KY (Louisville/Jefferson County Metropolitan Sewer District 2020). The removal of invasive species, restoration of streamside habitats, and planting of native species to support biodiversity and prevent erosion are central to these restoration projects (Louisville/Jefferson County Metropolitan Sewer District 2020).

Figure 4. The beginning stages of a stream and wetland restoration project along the Middle Fork of the Beargrass Creek in Louisville, Kentucky. This restoration will lessen urban flooding during extreme rain events.

Of all of the solutions to the urbanization of watersheds, wetland restoration is one of the most challenging (Zalidis et al. 1999). However, restoring wetland functions provides immediate benefits, from a reduction in pollution to an increase in biodiversity (Zalidis et al. 1999). Wetland restoration can even provide urban centers with economic benefits through a reduction in the costs associated with flooding and pollution control (Zalidis et al. 1999).

One of the challenges facing efforts to mitigate the impacts of urbanization on watersheds is the public perception of the problems and the willingness to allocate funding to these projects. In areas where the protection of water and avoidance of health and environmental risk is strongly valued, the public tends to be more willing to fund projects to reduce risks (Veronesi et al. 2014). Perceptions regarding climate change significantly impact willingness to fund restoration and mitigation projects (Veronesi et al. 2014). Public education will prove invaluable as climate change impacts grow, and urbanization threatens our most valuable and scarce resource - fresh water.

Literature Cited

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  1. Climate Change Brings Increasing Flood Risks to Louisville.

Metropolitan Sewer District. Climate Change Increasing Flood Risks as Louisville’s Protection Decays. WFPL. Available from https://wfpl.org/climate-change-increasing-flood-risks-as-louisvilles-protection-decays/ (Accessed July 2021).

  1. Beargrass Creek watershed stream daylighting project.

Brennan, Erin. “Louisville stream daylighting project.” 2021. JPEG file.

  1. MSD construction of Portland Combined Sewer Overflow (CSO) Basin to prevent overflow of sewage into local waterways. TBM Staff. 2019. Louisville (KY) MSD Portland CSO Basin Now in Service. Tunneling Online. Available from https://tunnelingonline.com/louisville-ky-msd-portland-cso-basin-now-in-service/ (Accessed July 2021).

  2. Louisville MSD Beargrass Creek watershed wetlands restoration project.

Brennan, Erin. “Grinstead wetlands restoration project.” 2021. JPEG file.

The Impacts of Urbanization on Watersheds