Rainfall, snowmelt, runoff, impervious surfaces, storm drains, MS4 systems, pollutants, sediment, nutrients, flooding, erosion, green infrastructure, rain gardens, permeable pavement, and watershed protection
Stormwater
Stormwater is rain or melting snow that runs off land and hard surfaces, carrying water and pollutants through neighborhoods, pipes, streams, and watersheds.
What stormwater is
Stormwater is water from rain or snowmelt after it reaches the ground. Some soaks into soil, some evaporates, and some runs across roofs, streets, lawns, construction sites, parking lots, and bare ground toward drains or waterways.
Why cities change runoff
Urban areas often replace soil and vegetation with hard surfaces. That means less infiltration, faster runoff, higher peak flows, and more pressure on storm drains, culverts, streams, and downstream floodplains.
What runoff can carry
Stormwater can pick up sediment, oil, metals, road salt, fertilizers, pesticides, bacteria, trash, tire particles, and other contaminants. The mix depends on land use, maintenance, weather, soil, and the first flush of runoff after dry periods.
Storm drains and pipes
Many neighborhoods collect runoff in storm drains, pipes, ditches, and outfalls. In separate storm sewer systems, this water may discharge to local streams, lakes, wetlands, or coastal waters without going through a wastewater treatment plant.
Flooding and erosion
Fast runoff can overwhelm drainage systems and raise stream levels quickly. It can also erode streambanks, scour habitat, move sediment downstream, and damage roads, basements, yards, and public infrastructure.
Green infrastructure
Rain gardens, bioretention areas, trees, vegetated swales, wetlands, green roofs, rain barrels, and permeable pavement can hold, slow, cool, and filter stormwater. These tools work best when matched to local soils, slopes, rainfall, and maintenance capacity.
Rules and planning
Stormwater programs may use permits, construction site controls, street sweeping, public education, erosion controls, maintenance standards, and development rules. The goal is to reduce pollutants and manage runoff before it becomes a downstream problem.
Why it matters
Stormwater links everyday surfaces to water quality, flooding, stream health, and drinking-water sources. Managing it well helps communities adapt to heavier rain, protect watersheds, and make streets and neighborhoods more resilient.