Urban Area Maps for NPDES MS4 Phase II Stormwater Permits

The universe of regulated small MS4s automatically expands every 10 years based on where population growth has concentrated in certain areas of the country as identified by each decennial census. This page provides information on the U.S. Census data that is used to identify newly designated small MS4s.

In a June 2023 final rule (pdf) (239.9 KB), EPA clarified that new small MS4 designations will be based on whether the previously unregulated MS4s are located in urban areas with a population of 50,000 or more people. The clarification was necessary because the designation criteria in the Phase II regulations had previously been based on the location of “urbanized areas” (defined as urban areas with a population of 50,000 or more people), and the Census Bureau announced in March 2022 (pdf) (286.6 KB) that it would no longer provide mapping information on such urbanized areas. The final rule ensures that the automatic designation criteria for MS4s remains the same as it has since the start of the Phase II program.

2020 Urban Areas with a Population of 50,000 or More People

To identify the location of newly designated small MS4s using the 2020 Census data, it is necessary to know both where the current urban areas are located and their associated populations. Knowing these two pieces of information enables the identification of urban areas with a population of 50,000 or more people. Use the following links to access the Census Bureau’s 2020 Census urban area maps and population data:

• Census 2020 Urban Area Reference Maps – The urban areas are shown in blue on the map, which can be zoomed into to locate your state and/or urban area of interest.
• Census 2020 Urban Area Population Data (Excel) – The population data are contained in an Excel file which lists the urban areas and their respective populations per state.
• Frequently Asked Questions (FAQs) (pdf) (277.94 KB)– The Census Bureau includes responses for FAQs regarding urban areas.

2010 Urbanized Areas

As noted above, prior to the 2020 Census, the criteria for automatic designation were based on Census-defined urbanized areas. For reference purposes, the location of urbanized areas in prior censuses, including the 2010 Census, can be found on the Census Bureau’s website. 

Each urbanized area folder contains a series of Portable Document Format (PDF) files:

• The first PDF contains a reference map and additional detailed maps.
• The remaining PDFs are individual copies of the detailed maps.

Note: The folders for some smaller urbanized areas contain only one map.

For reference maps of each urbanized areas from the 2010 Census, use the following website:

• Census 2010 Urban Area Reference Maps - The urbanized area and urban cluster both appear in green on the reference maps, which makes it difficult to distinguish them. However, the urbanized area labels appear in regular font while the urban clusters are slightly italicized.

The Census Bureau provides additional helpful information:

• Identify the correct urban area using a series of files relating the 2010 urban areas to counties, incorporated places, and zip codes.
• The original GIS shapefiles used to generate the maps can be downloaded under “boundaries and shapefiles” on the Census 2010 Urban Area website.