While winter application of deicers to paved surfaces have long been recognized as a key contributor to freshwater salinization in snow belt regions, studies investigating chloride (Cl) concentration ([Cl]) dynamics using high-frequency data are far fewer in Canada than in the United States (U.S.). Leveraging 15-minute measurements of specific conductance (SC) and bi-weekly grab sampling for [Cl], we derived continuous [Cl] for nine streams in urbanizing watersheds around Hamilton, Ontario, over the period of May 2020 to April 2021. By evaluating high-frequency [Cl] dynamics, we characterize dominant transport pathways and exceedances of water quality guidelines (WQG) to assess ecological risk. Specifically, we compare Canadian and U.S. WQG with respect to their [Cl] thresholds, and their duration criteria. Annual exceedance of the long-term Canadian WQG was >90% in six of the nine streams, highlighting the alarming extent of deicer-driven salinization in southern Ontario. Consistently-elevated [Cl] in the non-salting months indicates groundwaters supplying baseflow are Cl-impacted. Fast pathways (e.g., sewers) drive episodic flushing and dilution in the salting and non-salting seasons, respectively. Application of the U.S. WQG and its duration criteria for the long-term guideline (4-day daily rolling mean) conceals the extent of episodic dilutions which lower [Cl] below guideline thresholds. Future work is needed to determine whether these short-term [Cl] dynamics could provide brief periods of refugia for organisms from the impacts of Cl. While exceedance quantification is dependent on the guideline methodology, high-frequency data informed on transport pathways and ecological risk in southern Ontario streams.