EVIDENCE FOR THE 1927 MISSISSIPPI RIVER FLOOD IN A COASTAL FRESHWATER MARSH, LOUISIANA, USING PB-210 DATING

The Mississippi River experienced a 100-year flood event in 1927 with a discharge of about 72,000 m³/sec during the peak flow. This flood breached man-made containment levees in multiple locations and was responsible for flooding over 67,300 km² of the Lower Mississippi River Valley. In April, 1927, river stage at New Orleans was critical and a levee breach to the city was imminent. A levee south of the city was purposefully dynamited to alleviate pressure on the New Orleans levee, thereby flooding the Breton Sound estuary. Discharge through the breach reached 9600 m³/sec and lasted for 3-4 months. Geochronolgical dating evidence and a systematic survey of sediment deposits in the estuarine marshes demonstrate unequivocally the presence of the 1927 sediment flood layer. This layer appears as a wedge of deposition which decreases in thickness with increasing distance from the historic levee break location. The layer ranges up to 30 cm thick with an organic matter content of about 3%. Using ²¹⁰Pb dating, sedimentation rates change through time in the marshes. Deposition rates are 0.05 cm/yr today and show a change in sedimentation rate to 0.20 cm/yr before 1930. Marsh vegetation also appears to have shifted towards a more freshwater flora after the 1927 flood.