2.0 The Chenier Plain Ecosystem 



2.1 INTRODUCTION 



The Chenier Plain ecosystem is a rich and complex 

 mixture of wetlands, uplands, and open water that 

 extends about 322 km (200 mi) from Vennilion Bay, 

 Louisiana to East Bay, Texas (fig. 2-1). The 

 lengthwise boundaries cf the ecosystem are the 

 9 m (30 ft) depth contour along the sliore of the 

 Gulf of Mexico and the 1.5 m (4.9 ft) land elevation 

 contour. These boundaries are separated by distances 

 ranging from 16 km (10 mi) to 64 km (40 mi) and 

 encompass a total area of over 1,295 km (5,000 

 mi^). Several systems of rivers and lakes cross the 

 Chenier Plain from north to soudi and divide it into 

 six fairly distinct drainage basins. 



Pleistocene-age deposits, which forni the geologic 

 substrate of the Chenier Plain region, are found at 

 the surface a few kilometers inland from the coast 

 and dip gently seaward to .include the slope of the 

 Continental Shelf which is delineated by the 10 m 

 (33 ft) bathymetric line that lies about 8 km (5 mi) 

 offshore and by the 20 m (66 ft) line lying some 

 45 km (28 mi) offshore. These Pleistocene deposits 

 are ovedain at the coast by geologically Recent 

 sequences of inland stranded beaches that align the 

 topographic grain parallel with tlie coast. Near sea 

 level marslies interlaced with tidal channels lie between 

 successive ridges. The coastline is breached by inlets 

 tliat connect estuaries extending inland up river 

 basins. The exception is the East Bay Basin in Texas, 

 whose long axis parallels the coast. Although geo- 

 graphically part of the Chenier Plain, the topography 

 of this basin is similar to that found in basins of the 

 Strand Plain ecosystem to the west. 



Water- riverine, Gulf, and subsurface-is the 

 single most important medium for transporting 

 and mixing sediment, and nutrients. Rivers 

 function as arteries transporting sediments and 

 nutrients from inland catchrnent basins to the mixing 

 and receiving basins of estuaries, marshlands, and the 

 Gulf of Mexico. 



Meteorological forces interact with tides and 

 waves to generate currents along the coast and in 

 estuaries. Although highly variable from year to year, 

 climate exerts a long-term influence sustaining major 

 repetitive water movement patterns. Onshore winds 

 associated with summer sea breezes and offshore 

 winds that accompany the passage of winter cold 

 fronts raise or lower water levels, and drive surface 

 water. 



Landforms and accompanying habitats result 

 from the complex interaction, through time, of 

 geological, hydrological, and meteorological processes. 

 Parts 2.2 through 2.5 focus on these processes which 

 are relevant to understanding the development, 

 variability, and interaction of habitats. The basins 

 that compose the ecosystem function as discrete 

 units but are also subject to similar regional forces. 

 These basins, as the primary functioning units of 

 this study, are discussed in part 3.0. 



2.2 GEOLOGICAL PROCESSES 



Tliis section discusses the sedimentary and 

 erosional processes associated with land gain or loss, 

 and habitat development. Changes that have occurred 

 since the sea reached its present level, approximately 

 3,000 to 4,000 years ago are of primary concern. 

 The relation between events that occurred during the 

 last few decades and those presently underway are 

 also of significance. However, this record is framed 

 against coastal plain development processes (e.g., 

 alluvial, deltaic, and marine sedimentary processes) 

 that occurred during the last ice age when the sea 

 level was dramatically lower, as well as during the 

 time when the sea was rising to its present level. 



2.2.1 SEA LEVEL CHANGES 



The last continental glacial advance lowered the 

 sea level approximately 135 m (443 ft) below its 

 present level (Fisk and McFarian 1955, Gould 1970), 

 and the shoreHne was at a point approximately 200 km 

 (124 mi) seaward of its present position (RusseU 

 1936). Receding seas exposed the Pleistocene surface 

 (known as "Prairie" in Louisiana and "Beaumont" in 

 Texas) to erosion and weathering. With lowered 

 base levels, coastal streams along the Chenier Plain 

 cut valleys into the Pleistocene deposits (plate 2). 

 Subsequently during sea level rise with glacial retreat, 

 sequences of sediments were deposited on the eroded 

 Pleistocene surface (Saucier 1974). These sediments 

 consisted of sequences of open Gulf, bay, lake, marsh, 

 and swamp deposits. Each habitat can be described 

 from borings by the composition of flora and fauna 

 and the quantities and bedding characteristics of 

 sands, silts, and clays contained in the deposits 

 (Byrne et al. 1959). The depositional phase ceased 

 when the sea reached its approximate present level. 

 At that time the shoreline was landward of its present 

 position, as evidenced by the inland location of 

 former beach ridges of Recent age (fig. 2-2a and b). 

 Subsequently the shoreline advanced, by sediment 

 accretion, some distance seaward of its present loca- 

 tion. At present much of it is retreating again. The 

 entrenched valleys that were drowned during sea 

 level rise have not been filled with sediments but 

 form shallow inland lakes. 



2.2.2 LAND SUBSIDENCE 



Wlren combined with wave attack, loss of sedi- 

 ment supply, and sea level fluctuations, land subsi- 

 dence occurs. Tills process is lughly complex and 

 includes regional crustal downwarping of the Gulf 

 coast geosynchne, tectonic processes of folding and 

 faulting, and compaction of sediment tlirough de- 

 watering. Compaction, which is the major cause of 

 land subsidence, includes: differential consolidation 

 because of sediment textural variability; consolida- 

 tion of underlying sediments from weight of levees 

 (natural and artificial), beaches, buildings, piles, and 

 fills; lowering of the water table through extraction 

 of ground water, salt, sulfur, oil, or gas, or reclama- 

 tion practices; and extended droughts and marsh 

 burning that cause surface dehydration and shrink- 

 age in highly organic soils. 



