<p><strong>Abstract.</strong> We present a 3D unstructured-grid hydrodynamic model for the northwestern Gulf of Mexico that utilizes a high-resolution grid for the main estuarine systems along the Texas-Louisiana coast. This model, based on the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM) with hybrid horizontal and vertical grids, is driven by the observed river discharge, reanalysis atmospheric forcing, and open boundary conditions from the global models. The model

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... produces well the temporal and spatial variation of observed water level, salinity, temperature, and current velocity both in Galveston Bay and on the shelf. We apply the validated model to examine the remote influence from large rivers, specifically the Mississippi and Atchafalaya rivers, on the salinity regime along the Texas coast. Numerical experiments reveal that the Mississippi-Atchafalaya discharge could significantly decrease the salinity on the inner shelf along the Texas coast and its influence highly depends on the wind field and the resulting shelf current. Winter wind tends to constrain the Mississippi-Atchafalaya water against the shore, forming a narrow lower-salinity band all the way to the southwestern Texas coast. Under summer wind, the influence of the discharge on salinity is limited to the upper Texas coast while extended offshore. The decrease in salinity at the mouth of Galveston Bay due to the Mississippi-Atchafalaya discharge leads to a decrease in horizontal density gradient, a weakened estuarine circulation inside the bay, a decrease in the salt flux, and a smaller estuarine-ocean exchange. We highlight the flexibility of the model that simulates not only estuarine dynamics and shelf-wide transport but also the interaction between them.</p>