The AGS Globe: How Urban Landscapes Shape Runoff and Water Pollution

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By Betty Huang

As urban development increases, impervious surfaces such as roads, rooftops, and parking lots prevent rainfall from infiltrating into the ground. When it rains, water flows over surfaces, picking up pollutants and carrying them downstream into nearby waterways. In the United States, polluted runoff is the leading cause of water pollution, often including contaminants such as oil, heavy metals, fertilizers, bacteria, and trash. Classified as a nonpoint source, it comes from many diffuse sources, making it difficult to regulate since there is no single identifiable culprit. On the other hand, point sources refer to any single, identifiable, and localized origin of pollution, including any pipe, channel, well, concentrated animal feeding operation, or vessel from which pollutants can be discharged.

The most common urban pollutants include trash and litter, nutrients like nitrogen and phosphorus from fertilizers, sediments from soil erosion, bacteria from pet waste, metals from vehicles, and pesticides from lawns. Once transported into waterways, these pollutants begin to affect both water quality and ecosystem health in various ways. For example, when trash and litter accumulate, they can harm wildlife through ingestion and entanglement. Excess nutrients often lead to eutrophication, triggering algal blooms that reduce oxygen levels and negatively impact aquatic life. Sediment runoff increases water turbidity, obstructing sunlight and reducing photosynthesis of aquatic plants. Bacteria from pet waste pose health risks, frequently resulting in beach closures. Meanwhile, metals are toxic to aquatic organisms and can accumulate in the food chain, and pesticides can disrupt ecosystems even at low concentrations.

A key feature of urban runoff is the “first flush” phenomenon, where the initial rainfall after a dry period carries the highest concentration of pollutants. During dry seasons, contaminants accumulate on roads and surfaces. When rain falls, these pollutants are rapidly washed into storm drains and waterways, resulting in a concentrated dose of contaminants at the start of a storm event.

Most urban stormwater systems are designed primarily for flood control rather than treatment. Unlike household wastewater, stormwater runoff is discharged without treatment into nearby waterways, often called receiving waters. Receiving waters are any lake, river, stream, or ocean that accepts treated or untreated wastewater, effluent, or stormwater runoff. They are the final destination for pollutants from industrial, agricultural, and urban sources. These stormwater systems are part of a larger process known as a watershed, where water and pollutants move across the landscape before reaching these receiving waters.

We all live within a watershed. Watersheds are natural land areas that channel rainfall and snowmelt from higher elevations to lower elevation outlets such as rivers, lakes, or the ocean. The United States has 21 major watersheds and thousands of smaller sub-watersheds. Also known as drainage basins, watersheds are defined by their point of outflow. All land that drains water to that point makes up the watershed for that location. Water moves through watersheds primarily by gravity, flowing through connected streams and rivers. Because water carries materials with it, anything introduced into the landscape, including pollutants from urban or agricultural areas, can be transported downstream into receiving waters.

The largest watershed in North America is the Mississippi River Basin (lime green), which drains into the Gulf of Mexico. Meanwhile, the West Coast (red) drains into the Pacific Ocean, while the East Coast (green) drains into the Atlantic Ocean.

Any watershed with more than 5-10% impervious surface area is considered impaired. These surfaces act as “fast lanes” that transport the water directly into storm drains. Throughout the US, thousands of water bodies are classified as impaired as a result of stormwater-related pollution. These impaired waters require a Total Maximum Daily Load (TMDL), which sets the maximum pollutant level a water body can receive while still meeting water quality standards.

Urban stream syndrome is a common experience across almost all urban watersheds and their receiving waters. It refers to the degradation of streams in urban areas, where ecosystems are altered and stressed by increased inputs of waste and pollutants. This condition is largely driven by increases in rain and impervious surface cover, which alters the natural hydrology of watersheds. Change in land use from natural to built environments is also a big driver for overloading these systems.

These impacts often extend beyond individual streams, affecting entire watershed systems and downstream environments. Degraded water quality, increased flooding, and habitat loss reduce the ecological function of aquatic systems while also limiting human uses such as recreation, fisheries, and drinking water supplies. The most effective solutions combine green infrastructure, stormwater capture, reduced impervious surfaces, and pollution prevention to slow runoff, filter contaminants, and restore more natural watershed processes.

References

• U.S. Geological Survey (USGS). (2023). Stormwater runoff in urban watersheds [Image]. https://www.usgs.gov/media/images/stormwater-runoff-urban-watersheds

• Natural Resources Defense Council (NRDC). (2023). Water pollution: Everything you need to know. https://www.nrdc.org/stories/water-pollution-everything-you-need-know#categories

• The Nature Conservancy. (n.d.). Stormwater heatmap tool tracks pollution design thinking. https://www.nature.org/en-us/about-us/where-we-work/stormwater-heatmap-tool-tracks-pollution

• Mississippi Watershed Management Organization (MWMO). (n.d.). Watershed and stormwater basics. https://www.mwmo.org/learn/watershed-and-stormwater-basics/

• National Oceanic and Atmospheric Administration (NOAA). (2024). What is a watershed? https://oceanservice.noaa.gov/facts/watershed.html

• U.S. Environmental Protection Agency (EPA). (2025). Combined sewer overflows (CSOs). https://www.epa.gov/npdes/combined-sewer-overflows-csos

• Pennsylvania State University Extension. (2025). What is sediment, and why is it a stormwater pollutant? https://extension.psu.edu/what-is-sediment-and-why-is-it-a-stormwater-pollutant

• U.S. Environmental Protection Agency (EPA). (2026). Impaired waters and stormwater. https://www.epa.gov/tmdl/impaired-waters-and-stormwater

• University of California, Berkeley. (n.d.). Urban stream syndrome. https://creeks.berkeley.edu/creeks-and-watersheds/urban-stream-syndrome

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