Hydrology of the Gulf intra-coastal waterway in the San Bernard — Brazos river estuaries, Texas, USA: Oxygen isotopic ratio and salinity

A hydrological study using oxygen isotopic ratios and salinities was carried out in the San Bernard-Brazos River Estuaries, Brazoria County, Texas, USA, to understand the influence of 4 fresh water sources draining into the area, Brazos River, San Bernard River, Oyster Creek, and Jones Creek, on various parts of the Gulf Intra-Coastal Waterway (GIW) that transects these rivers half to 1 mile inland of the Gulf of Mexico. The measured oxygen isotopic ratios and salinities of GIW waters are explained with simple mixing between one of the 4 fresh water sources with marine water and made it possible to identify 3 subareas of the GIW, each of which is influenced by different fresh-water source. These sub-areas locate on the western part of the Brazos River, between the Brazos and Old Brazos River, and on the eastern part of the Old Brazos River, and have their freshwater source from the San Bernard River, Brazos River, and Oyster Creek, respectively. An interesting result was found in the GIW between the San Bernard and Brazos Rivers, where fresh water is mainly sourced from the San Bernard River although its discharge rate is 40 to 100 times less than that of the Brazos River. This westerly flow of the San Bernard River toward the larger Brazos River is due to shallowing of the San Bernard River at the river mouth, which restricts free discharge of river water to open sea and diverts the flow toward the GIW on both sides of the river. The influence of the other two creeks, Oyster and Jones, was minor and limited near the confluence with the GIW due to minor influxes. The generalized flow pattern derived in this study is essential for the construction of contingency plan against toxic contaminations, which will minimize environmental impact on numerous organisms living in the GIW. The results of this study are also useful for tracing of contaminants in the studied GIW.