Runoff from urban areas is a leading cause of pollution and decline in ecological health of downstream receiving waters. Mounting water quality issues and the price tags for clean ups have prompted a shift from gray stormwater infrastructure to “green stormwater infrastructure” (GSI) that provides ecological, social, and stormwater benefit. While all cities respond to the federal Clean Water Act, some cities have adopted GSI more than others. My research examines the role of federal stormwater regulation in shaping local policy, planning, and implementation, specifically for GSI. The summer of 2018, I conducted interviews with stormwater experts in the Los Angeles, San Francisco, Portland, and Seattle regions over summer of 2018. The results of my interviews suggest: 1) adoption of GSI is driven in part by the type of existing stormwater infrastructure, 2) high-achieving local municipalities influence state and maybe regional stormwater regulation, and 3) cities without a specific federal mandate to reduce stormwater volume face challenges to prioritize and evaluate trade-offs between GSI’s multiple functions.