Seasonal Runoff Variability as a Driver of Salt Wedge Propagation and Water Quality Dynamics in an Estuarine River System

This study aims to investigate the relationship between basin hydrology and estuarine processes such as dynamics that influence salinity and water quality in the Shatt Al-Arab River, southern Iraq. Extensive samplings were conducted at 25 sites along the river course over one hydrological year. Runoff estimates were obtained using the soil conservation service–curve number (SCS-CN) model. During winter, peak rainfall (76.8 mm month−1) and runoff (12.38 mm month−1) promote the shortest salt wedge extension (8 km) and the highest water quality (median water quality index (WQI) = 22). In contrast, during fall, minimal rainfall (6.51 mm month−1) and runoff (0.14 mm month−1) result in a salt wedge extension of 109 km and the lowest water quality (median WQI = 250). Strong correlations between rainfall–runoff estimates, salt wedge extension, and water quality parameters demonstrate that water quality status can be predicted using hydrological inputs alone. Thus, this study introduces a novel quantification of the flushing influence required to maintain the Shatt Al-Arab River’s ecological health. A strong (r2 = 0.87) significant (p < 0.05) negative correlation was detected between the runoff coefficient (a proxy indicator of catchment wetness) and the standard deviation of WQI. Such a negative correlation implies that hydrological flushing fosters water quality stability. Principal component analysis (PCA) further revealed how natural and anthropogenic sources contribute to water quality. The findings illustrate how seasonal hydrological variability control mixing processes, salt wedge propagation, and water quality in estuarine-influenced river systems, presenting a framework adaptable to similar systems worldwide.