Climate changes, including more frequent and intense storms and more extreme flooding events, can increase stormwater runoff. Stormwater runoff is rainwater or melted snow that runs off streets, lawns and other sites. When stormwater is absorbed into soil, it is filtered and ultimately replenishes aquifers or flows into streams and rivers.
An increase in stormwater runoff can exacerbate existing, or introduce new, pollution problems.
More frequent and intense downpours, projected for all regions of the country, can overwhelm the design capacity of municipal stormwater management systems. Overwhelmed stormwater management systems can lead to backups that cause localized flooding or lead to greater runoff of contaminants such as trash, nutrients, sediment or bacteria into local waterways.
More frequent and intense downpours can also challenge cities with combined stormwater and wastewater drainage systems. These systems can be overwhelmed by large amounts of rainfall or snowmelt and lead to more combined sewer overflows into waterways. An increase in combined sewer overflows can reduce water quality and make meeting water quality standards more difficult.
Stormwater runoff can also wash sediment, nutrients or other pollutants into water sources. Increased sediment, nutrients and other pollutants can diminish water quality, threaten drinking water sources, and complicate water treatment processes. Drought conditions and lower stream flow can exacerbate these problems by concentrating pollutants and limiting dilution.