National Geographic : 1983 Aug
decaying-comparisonsof its outline in 1956 and 1978 (below) give a shockingly clear account of the transformationof marsh, shown in green, to open water.Man-made controls upriver have decreasedthe amount of sediment carriedby the stream.Artificial levees along much of the lower Mississippikept flood-borne sediment from replenishingwetlands. In the active delta, rock sills installedacross breaks in the banks similarly confine the river.As a result an acceleratingrate of land loss indicates that most wetlands in PlaqueminesParish,for example, will be underwaterby the year 2000 unless more sediment-bearingriverwater is diverted into the marshes. Marsh 1978 Ocean intrudes as marsh sinks or washes away, no longer sustained by river sediment. Although the river still runs through the delta, the results of channelizing the stream have wrought damage comparable to a change Sin the river's course. Pea Barti Delta front Prodelta 1956 Although already less than the peak size reached in the 1800s, wide marshes still extend beyond the levees. Nature has heightened the levees, while man has deepened the river's channels and cut off marshes, leaving them susceptible to erosion and soil compaction. Peat Sunken marsh Bar Bar Man-made canals ~lldlt".