Polluted stormwater runoff to our lakes, rivers, and creeks is one of the leading sources of pollution in the Puget Sound region. The City of Seattle Stormwater Code guides private development in managing stormwater on the project site, which starts with the On-site Stormwater Management (OSM) requirement. This factsheet provides an overview and answers common questions about managing stormwater on commercial, institutional, industrial, or large residential developments.