Geochemical reactions occurring in estuarine waters can significantly affect the riverine flux of nutrients, minor elements, and trace elements to the ocean. Biological uptake, adsorption/desorption reactions, coagulation, as well as precipitation of iron hydroxides and high-molecular- weight organic acids, all can affect the chemical flux to the marine environment (e.g., Boyle et al., 1977; Sholkovitz et al., 1978; DeMaster et al., 1986). Many studies investigating these chemical reactions have been conducted in the estuaries of small rivers (discharge < 1014 1 y–1) because of simplicity in modeling the hydrographic regime or because of logistical reasons. Geochemical studies in these small estuarine systems have provided valuable insight to the chemical reactions that occur during the mixing of riverine and oceanic waters. However, much of the chemical flux from rivers enters the ocean via large dispersal systems. For example, the Amazon River alone supplies 17% of all the dissolved silica reaching the marine environment via rivers (Livingstone, 1963). Estuarine processes occurring in the more frequently studied small dispersal systems are extrapolated to those occurring in the large dispersal systems, but the validity of this extrapolation is unclear.