The early life stage effects of urban runoff (rain and snowmelt) on hatching, growth and survival of the fathead minnow(Pimephales promelas)was assessed in static and flow‐through systems. The data indicate a Maximum Allowable Toxicant Concentration (MATC) and a reduction of growth to 50% of controls at 28% and 60% runoff, respectively. Hatchability and average lengths were not as sensitive indicators of stress as millimeters produced per treatment. The data indicate a maximum toxicity in the fall which coincided with the reported drop in macroinvertebrate diversity during the same period, when untreated runoff events can contribute up to 1/4 of the river flow.