Increasing chloride concentrations in surface water and groundwater in Canada and the northern United States have led to concerns over freshwater salinization and subsequent impacts on aquatic ecosystem health. The issue is critical in the Greater Toronto Area where many freshwater streams exceed the long-term (chronic) water quality guideline for chloride. In this study, we quantified chloride concentration trends at 47 stream monitoring stations between 2000 and 2021 within the Toronto region and identified factors, such as chloride sources and changes in land use and climate, that may be contributing to the trends. Chloride concentrations increased significantly at 36 of 47 stream monitoring stations with 24 stations showing increasing trends during the winter (salting) season and 30 stations showing increasing trends during the summer (non-salting) season. Chloride-to-bromide ratios indicated that road salt and/or septic effluent was the dominant source of chloride; however, basin brines and/or animal waste, precipitation, or pristine aquifers also contributed chloride. The distribution of chloride sources varied by season with road salt and/or septic effluent dominating in the winter compared to a combination of basin brines and/or animal waste, landfill leachate, and pristine aquifer in summer. Change in road density was positively related to increasing chloride concentration trends; however, variability in winter severity was not. The results of this study highlight the usefulness of long-term water quality, land use, and climate data for understanding the drivers of freshwater salinization and point to the need for better tracking of chloride inputs (e.g., winter salt) to the landscape.