<p>Understanding estuarine dynamics in macrotidal estuaries is challenging, especially in large-scale systems, where the physical processes are complex and not well studied. This study evaluated the response of thermohaline structure and suspended particulate matter (SPM) concentration to variations in river discharge in the São Marcos Estuarine Complex (SMEC), a large-scale, macrotidal tropical estuary on the Amazon coast. Oceanographic observations were made during the dry (September 2023) and rainy (May 2024) seasons during the spring and neap tides, when the salinity, temperature, turbidity, SPM concentration, currents, and water level in the estuary were measured. The results indicate that the SMEC is predominantly well-mixed in both seasons. Tidal propagation is asymmetric and hypersynchronous, generating strong currents and high levels of vertical instability, which induce intense mixing in the water column. The currents exhibit asymmetry in the middle and upper estuary, with a predominance of flood tide in the dry season and ebb tide in the rainy season. Throughout the estuary, the SPM concentration in the water column is mainly sustained by resuspension of the bottom material, with higher concentrations in the dry season. The location of the turbidity maximum zone (TMZ) in the middle estuary (~ 90&#xa0;km from the mouth) indicates that tidal intrusion acts as a natural barrier, preventing higher SPM concentrations from advancing to the lower estuary. The strong tidal activity in the SMEC offers valuable insights into estuarine dynamics and SPM variability, in addition to contributing to predictions of responses to future environmental changes in large-scale and macrotidal estuaries.</p>