Chloride from deicing salts in urban runoff can harm aquatic life. Stormwater ponds can help manage this, but their impact on chloride levels is not well understood. This study measured chloride levels in two ponds in Ontario. One pond consistently kept chloride, while the other sometimes released it. Downstream of the ponds, chloride levels were often too high for aquatic organisms, especially during releases from the ponds. This shows that stormwater ponds can concentrate and change the timing of chloride releases into streams, making areas downstream more at risk of chloride toxicity. These findings will help improve models of how chloride moves through watersheds and identify areas vulnerable to salt pollution.