Freshwater salinization resulting from winter road salt application is a ubiquitous environmental concern in seasonally frozen environments. While salt management plans are increasingly implemented by municipalities, predictive models for stream chloride (Cl) often lack a holistic integration of shifting watershed conditions. Current research gaps persist regarding how the synergy of rapid urbanization, climate change, and legacy Cl—salt sequestered in soil and groundwater—will dictate future water quality. Furthermore, the specific influence of high-frequency freeze-thaw events on salt transport remains largely absent from predictive modeling frameworks. This study introduces an integrated hydrology and water quality modeling approach using the Storm Water Management Model (SWMM) applied to a highly urbanized watershed draining into Lake Ontario. To address the complexities of salt transport, we employ a factorial design framework that evaluates the individual and combined effects of projected land-use changes, future climate scenarios, legacy salt contribution, and salt reduction targets. A key point in this research is the explicit calibration of the model to account for freeze-thaw cycles, providing a timely analysis of how abrupt, extreme winter events may alter Cl dynamics. Additionally, we quantify the escalating contribution of legacy chloride by analyzing 50 years of historical stream Cl trends during low-flow conditions. By synthesizing these variables, this research aims to provide a comprehensive predictive tool for understanding how current and future watershed conditions will influence long-term salinization trajectories, offering vital insights for sustainable urban salt management.