MODELING SEDIMENT DELIVERY TO LOUISIANA COASTAL 

 SALT MARSHES: NATURAL PROCESSES AND OPTIONS 



FOR MANAGEMENT 



Denise J. Reed 



Louisiana Universities Marine Consortium 



Chauvin, LA 70344 



ABSTRACT 



Loss of Louisiana's coastal salt marshes is frequently attributed to insufficient sedimentation to 

 keep pace with the rapidly rising sea level. Loss of sediment input caused by controlling Mississippi 

 River flow has reduced natural sedimentation rates but some sediment, both organic and inorganic, 

 is still being actively deposited on the marsh surface. This study attempted to elucidate the natural 

 process of sediment delivery to the marsh surface and identify the role of events of differing 

 magnitudes in contributing sediment. Sediments samples were collected and water levels monitored 

 during the winter and spring of 1987 near the Louisiana Universities Marine Center in Cocodrie, 

 LA Short-term sediment deposition rates are compared to the amount of water flooding the marsh 

 surface during monitoring periods. The results show a clear relationship between the amount of 

 sediment deposited and the depth and duration of marsh flooding. Regular astronomical tidal 

 flooding contributes very little sediment to the marsh compared to flooding associated with the 

 passage of a cold front. The identification of this relationship allows existing tide-gauge records to 

 be used to model the development of the present marsh, in terms of the frequency and magnitude 

 of sediment deposition. 



Plans to manage and restore salt marshes subjected to sedimentation deficit frequently involve 

 increasing the sediment input to the marsh areas. The results of this study show that sedimentation 

 may not be limited by sediment availability but by mobilization and mechanisms of delivery to the 

 marsh surface. Increasing available sediment in coastal salt marshes will not necessarily promote 

 increased sedimentation, which will be dependent upon natural sediment delivery processes unless 

 marsh flooding is also manipulated. Therefore, large-scale management plans for Louisiana's salt 

 marshes should consider both the input of sediment necessary to overcome the sedimentation deficit 

 and mechanisms of sediment distribution. 



INTRODUCTION 



One of the main causes of Louisiana's wetland loss, currently estimated at 0.86% per year (Turner 

 and Cahoon 1987), appears to be insufficient sedimentation on the marsh surface. This sediment 

 is required to maintain the marsh surface elevation in the face of a rising relative sea level and to 

 provide nutrient stimulus to plant growth, itself a contributor to marsh accretion and soil 

 development. Although studies have identified the marsh surface sedimentation in many areas as 

 insufficient for these needs, some sedimentation is still occurring on the marsh surface in saline, 

 brackish, and fresh environments. These sediments are being transported onto the marshes by 

 natural processes, frequently operating in channels and canals altered by humans, and in areas which 

 are not actively being managed. 



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