Dynamics of intertidal marshes near shallow estuaries in Louisiana

A three-year (1991–1993) field investigation was conducted to quantify the hydrodynamics of intertidal marshes adjacent to tidal channels and shallow bays within two Louisiana coastal regions: (1) the sediment-rich Atchafalaya Basin, and, (2) the sediment-poor Terrebonne Basin with relatively minor riverine inflow. The Terrebonne Basin marsh is regularly inundated and flooding is characterized by sporadic draining interspersed by prolonged flooding events. The maximum water depth on the marsh surface exceeds 50 cm, the flow velocity across marsh surface reaches 10 cm sec^–1, and the sediment deposition rate varies from 10 to 90 g m^–2 per tidal cycle. This rather high sediment deposition rate occurs during winter storms with strong southerly winds. In contrast, the marsh site within the sediment-rich Atchafalaya Basin is irregularly inundated and characterized by sporadic flooding interspersed by prolonged draining. There the marsh flooding depth rarely exceeds 25 cm, the over-marsh flow velocity barely reaches 2.5 cm sec^–1, and the sediment deposition rate ranges from 5 to 50 g m^–2 per tidal cycle. The surprisingly low rate of sediment deposition in a marsh within a sediment-rich region is largely due to the man-made canals that alter the hydrologic regime in the upper reaches of the tidal channel.