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Several studies have documented the use of pesticides in cannabis cultivation. In northern California, one of the top cannabis production regions, several studies have identified cannabis-related impacts on multiple terrestrial wildlife species. To date, research has not focused on the potential for cannabis-related pesticides to contaminate downstream waterways and aquatic species. We conducted a two-part multi-scale study utilizing polar organic chemical integrative samplers (POCIS) to monitor pesticide contamination (1) immediately downstream and upstream of illegal public land cannabis cultivation complexes in low-order tributaries and (2) below a gradient of private land cannabis cultivation operations within higher-order streams. Diazinon and carbofuran were confirmed within sensitive headwater streams downstream of illegal public land cultivation sites in remote settings within four National Forests. Diazinon demonstrated higher downstream transport potential, with overland and in-stream flow distances totaling up to 186 m downstream of cultivation areas. While carbofuran displayed greater temporal longevity, being detected over 490 days after the last estimated pesticide application, no positive detections were identified within POCIS deployed within higher-order catchments. The utility of targeted POCIS deployed within low-order catchments is validated and confirms downstream cannabis-related water contamination on National Forest lands. HIGHLIGHTS Foundational investigation of water quality impacts associated with cannabis cultivation.; Carbofuran and diazinon water contamination confirmed downstream of public land cannabis cultivation complexes.; Pesticides were detected at 11% of downstream sampling stations during the first wet season following site eradication.; POCIS utility confirmed for low-order headwater streams.;