Approximately 70% of China’s rivers and lakes are unsafe for human use. Effective implementation of existing pollution standards can improve the health and well-being of people across China. In this randomized trial, pollution decreased when a nongovernmental organization enlisted volunteers to monitor the quality of urban waterways slated for remediation and disseminated that information to local and provincial authorities. Disseminating information to the public through posters did not have detectable effects on water quality. Nongovernmental organizations can support the efforts of authorities to remediate pollution by providing monitoring that guides action and facilitates oversight between different levels of government, particularly when authorities that set remediation targets have an active interest in responding to public complaints and using monitoring for oversight.