Long-Term Multivariate Dynamics of Water Quality in the Chicago and Des Plaines River Watersheds: Evidence from Principal Component Analysis (2001–2025)

Urban freshwater systems are subject to complex, interacting anthropogenic stressors that collectively alter hydrological, chemical, and ecological dynamics. This study examines the temporal evolution of water quality across the Chicago River Watershed (CRW) and the Des Plaines River Watershed (DPRW) over a 25-year monitoring period (2001–2025). Long-term data from 51 stations were analyzed across ten water quality parameters. Principal component analysis (PCA) was applied annually to characterize shifts in multivariate water quality structure and identify dominant gradients governing system behavior. During the early phase (2001–2012), three principal components described the system, reflecting semi-independent stressor gradients. Beginning in 2013, a marked structural simplification emerged where a single dominant component accounted for approximately 78–84% of total annual variance, indicating strong parameter coupling and consolidation of system variability into a unified response gradient. This transition coincided with measurable declines in total phosphorus, total dissolved solids, and nitrogen. Nevertheless, the persistence of a single dominant gradient underscores the continued influence of urban environmental controls and tightly coupled pollutant pathways. These findings affirm the value of long-term, multivariate monitoring for characterizing urban water quality dynamics and informing adaptive watershed management.