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Hurricane Harvey (Harvey) struck the Texas coast 25–27 August, 2017, and produced massive flooding of the Galveston Bay tributaries. The slow release of floodwaters from the Barker and Addicks Reservoirs, west of Houston, produced a 48-day flood across metro-Houston through via Buffalo Bayou, through the San Jacinto Estuary (SJE), before flowing into Galveston Bay. To quantify the thickness and mass of sediment which was delivered to Galveston Bay as well as the mass of mercury (Hg), a series of 56 pushcores were collected from across Galveston Bay, including the San Jacinto Estuary, Trinity Bay, Galveston Bay, East Galveston Bay, Clear Lake, and Dickinson Bayou. X-radiographs of each core was used to quantify the thickness of the flood layer and average water content of the cores was used to estimate bulk density of the cores in the estimation the flood deposit mass. Using average Hg concentrations from each core, combined with the bulk densities, Hg mass calculations were made for the sub-bays and the entire bay system. Within Galveston Bay and the SJE, Harvey deposited 131.34 × 106 t of sediment, representing 31 years of average annual sediment load to the bay. The combined Galveston Bay and SJE Harvey deposit contains 5.09 t of Hg. This Hg was primarily sourced from legacy Hg which had been archived in lower Buffalo Bayou and the SJE due to elevated land subsidence and as a result of historical releases from industrial wastewater, primariy from Patrick Bayou, a small tributary of lower Buffalo Bayou. Harvey flooding eroded nearly the entire upper 0.5 cm of lower SJE, resulting in the erosion of 16.4 × 106 t of sediment, containing at least 3.43 t of Hg, from the SJE and Patrick Bayou, dispersing it within Galveston Bay. The floodwater during peak discharge rapidly exited Galveston Bay into the Gulf of Mexico. In contrast, the slow release from the Barker and Addicks Reservoirs resulted in prolonged hydraulic trapping of both floodwaters and suspended sediment within Trinity and upper Galveston Bay. The prolonged hydraulic trapping contributed to the broad dispersal of Hg across Galveston Bay and also increased the potential exposure of marine organisms to the elevated concentrations of Hg within the suspended sediment. With the increase in slow-moving storms and the archiving of legacy contaminants within urbanized estuaries, especially where there is elevated land subsidence, suggests that the impact and dispersal of legacy contaminants within Galveston Bay by Harvey is a harbinger of things to come for other urbanized estuaries throughout the world.