Equal volume surface sediment samples from the eastern delta margin were studied for Foraminifera. Six faunal assemblages, based on living specimens are defined: 1) marsh, 2) interdistributary bay, 3) fluvial marine, 4) deltaic marine, 5) sound, and 6) open shelf. Bottom-water masses, characterized by limits of variability of the chemical and physical properties, coincide with the faunal distribution. No single property of the bottom water appears to be the controlling ecological factor. There does not seem to be any causal relation between faunas and sediment characteristics; both appear to result from other processes active in a deltaic environment. Maximum living populations are associated with the high sedimentation rates of the prograding delta front. The smallest standing crops occur where sedimentation rates are low. The total number of Foraminifera per sample ranges from 150 to 34,000 specimens, and the maximum population zone is related to the area of highest foraminiferal productivity. Living and total counts are qualitatively similar except in areas where mud lumps containing relatively deep-water species are being eroded into shallow interdistributary bay deposits. Ratios of the living and total populations generally reflect effects of sedimentation.

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