Assessing mixed sediment sources of the brackish lakes of the Brazos-San Bernard River Delta of the northwest Gulf of Mexico and what it can tell us about how anthropogenic alterations have impacted sediment sources and sedimentation rates

This study examines the impact of human-induced changes to the lower Brazos-San Bernard delta on sedimentation rates and sediment sources filling the estuarine lakes within its western inland area. The delta began forming in 1929 when the Brazos River mouth was relocated 10 km west of its original position, placing it 5 km from the San Bernard River's mouth. Over time, the wave-dominated Brazos Delta expanded westward, closing the San Bernard River mouth and extending beyond it. 

In 1949, the Gulf Coast Intercoastal Waterway (GCIWW) canal was completed, bisecting the Cedar Lakes, a series of five brackish lakes, and enabling sediment transport to these lakes. The San Bernard River drainage basin, situated between the larger Brazos and Colorado Rivers, occasionally receives floodwaters from the Colorado River during high-discharge events like Hurricane Harvey (2017). The geological differences between the Brazos and Colorado Rivers facilitated the development of distinct sediment "fingerprints" using X-ray fluorescence, color spectrometry, imaging, and grain size analysis. 

Eighteen vibracores collected across the study area, combined with 137Cs dating, enabled the identification of deposits formed before and after the GCIWW's creation. Findings revealed that prior to the GCIWW and the Brazos River mouth's relocation, Colorado River-derived deposits dominated the region. Following these alterations, the western Cedar Lakes recorded Gulf of Mexico overwash sands and occasional Brazos River flood deposits. The GCIWW acts as both a conduit for Brazos River sediment and a barrier to Colorado River sediment. 

North of the GCIWW, in lakes isolated from the canal, sediment records show a mixture of Brazos and Colorado River deposits. East of the San Bernard River, deposits include Colorado River material, Gulf of Mexico overwash, or layered deposits from both the Brazos and Colorado Rivers, reflecting simultaneous flooding events. Most Cedar Lake cores exhibit Pleistocene deposits at their base, overlaid by 50–90 cm of pre-GCIWW sediments and 90–130 cm of post-GCIWW deposits. This suggests a significant increase in sedimentation rates after the canal's construction. 

In summary, the GCIWW's creation and the Brazos River mouth relocation have significantly altered sediment sources and deposition rates in the Cedar Lakes and adjacent brackish lakes of the Brazos-San Bernard delta.  The results of this study are being used by various stakeholders to develop a better management plan for this region.