Dynamics of the Turbidity Maximum Zone and Its Relationship with the Salt-Wedge Position in a High-Discharge Microtidal Estuary

The Magdalena River Estuary hosts the access channel to the Port of Barranquilla, where recurrent dredging is required to maintain navigable depths of up to approximately 12 m. Chronic siltation in this channel is closely linked to the dynamics of the Turbidity Maximum Zone (TMZ), which remain poorly understood in tropical, microtidal systems with extreme sediment loads. This study investigates the spatiotemporal variability of the TMZ in the Magdalena River Estuary (MRE), Colombia, using a previously calibrated and validated MOHID 3D numerical model coupled with sediment transport. Sixteen scenarios covering river discharges from 2000 to 5500 m3 s−1 under neap and spring tidal conditions were analyzed. Results show that the TMZ core position follows a nonlinear inverse relationship with discharge (R2 = 0.976), migrating from km 13–15 under extreme low-flow conditions (Q = 2000 m3 s−1) to the estuary mouth for discharges above 5000 m3 s−1. Within the simulated discharge range of 2000–5500 m3 s−1 and under the modeled neap and spring tidal conditions, the position where ε = 0.005 tracks the TMZ core location (R2 = 0.96, RMSE ≈ 1 km), suggesting that this threshold can be used as a first-order spatial indicator of maximum sedimentation under the conditions evaluated in this study. Contrary to macrotidal estuaries, the MRE exhibits higher suspended-sediment concentrations during neap tides than during spring tides, with SSC up to 77 percent greater for Q = 2000 m3 s−1. This reversal is driven by the suppression of turbulent mixing (Ri > 20) during neap conditions, which preserves the salt-wedge structure and enhances stratification-controlled sediment trapping. These results provide two process-based criteria for predicting turbidity-maximum behavior in the MRE: the ε = 0.005 stratification isoline and the discharge–TMZ polynomial. More broadly, the methodological framework may support the development of site-specific predictors for other highly stratified, microtidal estuaries subject to strong discharge variability.