Imidacloprid, a nicotine-based systemic insecticide, is one of the highest-selling pesticides globally. It is extensively used in both agricultural and urban settings. Neonic insecticides have been most prominently linked to colony collapse disorder in honeybees and pose a significant threat to aquatic invertebrates. The U.S. EPA benchmarks for freshwater species exposure to imidacloprid include a value of 385 ng/L for acute exposures and 10ng/L for chronic exposures. Within the United States, concentrations of imidacloprid detected in streams and rivers routinely exceed acute and chronic toxicity endpoints derived for freshwater invertebrates with positive correlation between percentage of urbanization and detection of imidacloprid. Forester Creek, a 16-km tributary to the San Diego River, was the study site of interest. It was chosen due to its previous wetland restoration project and its state of impairment for a variety of pollutants. Imidacloprid was detected with 100% frequency in surface water samples collected from Forester Creek with a median concentration of 16.9 ng/L (range: 3.8 to 96.8 ng/L). Over 60% of individual samples exceeded the U.S. EPA’s chronic exposure benchmark. Temporal analysis displayed significantly higher levels during wet weather events than dry weather events (median 45.6 ng/L vs. 8.2 ng/L (p < 0.05)), demonstrating the influence of urban runoff on stream quality. Imidacloprid generally followed a first flush pattern with the highest levels observed on the rising portion of the hydrograph as compared to the remainder of the storm again indicating that the build-up and wash off from land surfaces during storms is a major source of this insecticide. To our knowledge, the present study is the first to document this first-flush pattern for imidacloprid in an urban stream in southern California. The restored riparian/wetland section of Forester Creek did not demonstrate a significant removal of imidacloprid (median 12.6 ng/L [RD] vs. 24.9 ng/L [RU] (p > 0.05)). Such data on the occurrence and levels of imidacloprid in this urban stream contribute to the limited knowledge on imidacloprid in urban environments and will promote a better understanding of sources and effects of the neonicotinoid pesticide within the urban southern California region.