Urbanization's Impact: Streams And Pollution

Urbanization has a significant impact on stream ecosystems, and with 80% of Americans living in metropolitan areas, it is a pressing issue. As watersheds become covered in impervious surfaces like pavement and buildings, streams experience increased stormwater runoff, higher levels of fertilizers and pesticides, and changes to their physical habitats. These factors can lead to erosion, degradation of fish habitats, and water quality issues, affecting both the stream organisms and the humans who rely on these water sources. Understanding the effects of urbanization on streams is crucial for developing effective management strategies to protect and restore these valuable ecosystems.

Urbanization and stream hydrology

Urbanization affects stream hydrology in several ways. Urbanization alters the natural hydrologic regimes of streams, leading to changes in discharge patterns, such as increased surface runoff and decreased infiltration of precipitation. This can result in higher stream flows and erosion of stream beds and banks. Urbanization can also impact the timing and magnitude of precipitation, with urban areas experiencing increased rainfall due to the urban heat island effect.

The expansion of impervious surfaces, such as pavement and buildings, reduces the infiltration of water into the ground and increases stormwater runoff. This can lead to more frequent and intense flooding events. Additionally, urbanization can alter the chemistry and ecology of streams, including increased pollution from fertilizers, pesticides, and sewage.

The effects of urbanization on stream hydrology can vary depending on the specific geographical region and the extent of urban development. Some studies have shown that even small levels of urban development can have immediate negative effects on stream ecosystems.

Urbanization and water chemistry

Urbanization and population growth have had a significant impact on water chemistry and stream ecosystems. As watersheds become urbanized, the magnitude and pattern of response in stream hydrology, water chemistry, and biological communities are altered. This is due to the increased volume of stormwater runoff, exposure to fertilizers and pesticides, and changes in the physical living spaces within streams.

Urbanization can lead to an increase in storm water runoff as roads, parking lots, and buildings restrict the infiltration of precipitation into groundwater. This can cause stream flows to rise rapidly after rainfall, leading to erosion of stream beds and banks and degrading fish spawning and feeding habitats. Common sources of pollution in urban streams include fertilizers, pesticides animal wastes, sewage, erosion from construction, automobile fluids, and industrial emissions. These contaminants can have detrimental effects on aquatic life and downstream water quality, harming biological communities and economically valuable resources such as fisheries and tourism.

The U.S. Geological Survey (USGS) has conducted studies in several metropolitan areas across the United States to investigate the effects of urbanization on stream ecosystems. These studies have found that even small levels of urban development can have immediate negative effects on stream organisms and biological communities. For example, in Anchorage, Alaska, sensitive invertebrate communities declined when about 5% of the land cover in the watershed was converted to impervious surfaces.

The effects of urbanization on water chemistry and stream ecosystems are complex and vary depending on the region and the specific characteristics of the watershed, such as basin slope, geology, and soils. It is important to understand these effects to develop effective management strategies for protecting and restoring urban streams.

With the world's urban population continuing to grow, it is crucial to address the impacts of urbanization on water chemistry and stream ecosystems. Strong city planning and management of urban-related stressors will be essential to protect stream health and ensure the sustainability of urban areas.

Urbanization's impact on aquatic life

Urbanization has a detrimental impact on aquatic life, affecting both the organisms that live in streams and the wider biological communities that include rivers, reservoirs, and coastal waters. This is due to the contamination, habitat destruction, and streamflow flashiness that result from urban development.

Contamination

Urban development increases the volume of storm runoff, causing higher levels of water contamination. Common sources of pollution in urban streams include fertilizers and pesticides, animal waste, seepage from septic tanks, liquids from sewage lines, automobile fluids, and industrial emissions. These contaminants can cause harm to aquatic life, such as fish and other economically valuable resources like fisheries.

Habitat Destruction

The conversion of rural and forested lands to urban areas results in the loss of habitat and food sources for aquatic organisms. This can be further exacerbated by the presence of physical barriers, such as roads and parking lots, that restrict the infiltration of precipitation into groundwater, leading to increased stormwater runoff and erosion of stream beds and banks.

Streamflow Flashiness

The impervious surfaces that come with urban development, such as roads and buildings, alter the natural flow of streams. This can lead to flash flooding, which not only impacts aquatic life but also poses risks to human populations living in urban areas.

Loss of Urban Tree Cover

Pollution and physical barriers to root growth contribute to the loss of urban tree cover. Trees play an important role in maintaining water quality by absorbing excess water and filtering pollutants before they reach water bodies.

Management Strategies

To protect aquatic life, it is crucial to identify the specific urban-related stressors that contribute to biological community degradation. Management strategies may include strong city planning, upgrading energy use and transport systems, waste disposal solutions, and incorporating the care of green spaces into urban planning.

Urbanization and stream temperature

Urbanization can have a significant impact on stream temperature, leading to increased daily maximum temperatures and a higher number of temperature exceedances, particularly during the summer months. This phenomenon is often attributed to the development of urban heat islands, characterized by localized areas of heat storage and elevated air temperatures near urban centers. The following factors associated with urbanization contribute to the warming of streams:

• Riparian alteration: Changes to the riparian zone, such as a reduction in canopy cover and shading, can lead to increased solar radiation reaching the water surface, resulting in higher stream temperatures.
• Stormwater runoff: Warm impervious surfaces in urban areas contribute to heated surface runoff, while also reducing cooler groundwater inputs due to decreased infiltration.
• Lower baseflows: Shallower water levels caused by lower baseflows can lead to the formation of standing pools that warm up quickly, further elevating stream temperatures.
• Physical habitat modifications: Channel widening, for example, can increase the width-to-depth ratios of streams, reducing riparian shading and exposing more surface area for heat exchange, which intensifies warming.
• Urban stream management practices: Certain management strategies, such as the use of stormwater retention ponds, can inadvertently increase water retention time and warming, particularly if these ponds are unshaded.
• Wastewater inputs: The discharge of warmer effluents from wastewater treatment plants can directly contribute to higher stream temperatures.

The rise in stream temperature can have detrimental effects on aquatic organisms, including lethal and sublethal consequences such as increased disease susceptibility and decreased growth and reproduction.

Urbanization and water quality

Increased Stormwater Runoff and Erosion

One of the primary consequences of urbanization is the increase in stormwater runoff. As urban areas are covered with impervious surfaces such as pavements, sidewalks, and roads, rainwater is unable to infiltrate the ground and instead flows rapidly into streams, leading to erosion of stream beds and banks. This altered hydrology can degrade fish spawning and feeding habitats.

Water Pollution

Urbanization also contributes to water pollution through various sources. Industrial discharges, vehicle emissions, residential and commercial wastewater, trash, and the use of fertilizers and pesticides all contribute to the contamination of urban waterways. These pollutants can have far-reaching effects, impacting not only the urban streams but also larger water bodies downstream, including rivers, reservoirs, and coastal waters.

Changes in Water Tables

The extensive pavement and construction in urban areas reduce the amount of water soaking into the ground. This results in a lowering of the underground water table, causing some existing wells to run dry. Additionally, the increased use of large wells can further deplete groundwater reserves, leading to land subsidence and the intrusion of saltwater into drinking water sources.

Inaccessibility of Waterways

Urban development often makes waterways inaccessible to adjacent neighborhoods. This lack of access limits the community's ability to enjoy recreational activities, fishing, or the benefits of living close to water.

Positive Impacts

While urbanization often has negative consequences for water quality, proper management and maintenance can yield positive impacts. Improvements in storm drainage systems, wastewater reuse, and ecological recharge ponds can help reduce flooding and pollution, enhancing water conservation and the health of aquatic ecosystems.

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