As coastal areas become more vulnerable to climatic impacts, the need for understanding estuarine carbon budgets with sufficient spatiotemporal resolution arises. A mass balance model has been constructed for carbon fluxes in four estuaries along the northwestern Gulf of Mexico (nwGOM) coast from 2014 to 2018. The annual lateral carbon exports from tidal marsh-mangrove to estuaries account for 97.9% and 84.4% of total organic carbon (TOC) and dissolved inorganic carbon (DIC) inputs, respectively. This sustains a relatively high air-water CO2 flux (16.8 ± 3.0 mol·C·m-2·yr-1) compared with most estuaries on the North American Atlantic coast. In addition, annual air-water CO2 flux reaches as high as oceanic DIC export coastwide. The majority of imported riverine TOC has been exported to the coastal ocean (62.2%), leaving 22.3% of TOC for sediment deposition and 15.5% for remineralization. These fluxes are highly variable because of hydrologic variability. For example, episodic flooding can elevate estuarine CO2 efflux by 2 – 10 times in short periods of time. Flood following a drought state also increases lateral exchanges of TOC (from 90.7 ± 65.7 to 200.5 ± 160.2 mmol·C·m-2·d-1) and DIC (from 49.1 ± 39.8 to 166.9 ± 236.1 mmol·C·m-2·d-1). The contribution of nwGOM estuaries increases the overall North American estuarine CO2 flux by 220%, impacting coastal carbon budget. Hydrologic control explains temporal variability in these estimates.