3.0 Chenier Plain Basins 



3.1 INTRODUCTION-GENERALIZED BASIN 

 DESCRIPTION 



A basin, the result of long-tenn geologic pro- 

 cesses, can be described as a set of interacting habitats 

 constrained by climate and physiography, and inte- 

 grated by the flow of water througli it. Each habitat 

 type has characteristic species and productivities. 

 Man is a major factor in a basin; his activities influ- 

 ence nearly all natural processes. The objectives of 

 the basin-level analysis are to describe the natural 

 functions of Chenier Plain basins and the modifica- 

 tions caused by human activities. 



A conceptual model of basin-level processes 

 and interactions places in perspective the detailed 

 analysis that follows. The model (fig. 3-1) includes 

 only the most critical components and processes, 

 and shows interactions of water, wetlands, up- 

 lands, wildlife and fish, and man. At this level of 

 analysis, hydrological and land-modifying processes 

 are emphasized because they determine the capacity 

 of a basin to support renewable resources such as 

 waterfowl and fishes. Thus, the basin-level discussion 

 of living resources emphasizes factors tliat change 

 habitat area rather tlian habitat quality. The latter 

 is discussed in part 4. Differences among basins result 

 from differences in the relative areas of habitats, the 

 degree of interaction among them, and in basin inputs 

 and outputs. 



A drainage basin can be envisioned as four linked 

 submodels driven by a set of forces external to the 

 basin (fig. 3-1). Each submodel represents a different 

 group of processes interacting within the basin. They 

 are: (A) basin hydrologic processes, which represent 

 water storage and flow through a basin; (B) land- 

 modifying processes, which result in the exchange of 

 area among different habitats and especially in the 

 loss of natural wetland; (C) the renewable resource 

 productivity of a basin, or its capacity to sup- 

 port wildhfe and fish species, to purify water, and to 

 perfomi other services for men; and (D) basin-level 

 socioeconomic processes, those human activities and 

 management decisions that impinge directly on 

 natural processes in a basin. 



3.1.1 HYDROLOGIC PROCESSES IN A BASIN 



Hydrology (part 2.4) has already been identified 

 as a major factor in the development of the entire 

 Chenier Plain region and is largely responsible for the 

 unique characteristics of each basin. Further, the 

 hydrologic regime at any specific site within a basin 

 determines the kijid of habitat that develops at the 

 site (Bahret al. 1977). Basin hydrology results from; 

 the interactions among water stored and flowing in a 

 basin (Aj); upstream riverine and rainfall inputs of 

 water, sediment, and nutrients (Aj); and downstream 

 tidal water with accompanying salts, sediments, and 

 nutrients (A3) (fig. 3-1). 



Hydrology detennines habitat type by water 

 levels, flows, and salinity gradients. Water levels are 

 controlled by the pressure head between water level 

 at a given site and upstream and downstream water 

 levels; consequently they are modified by rainfall, 

 tidal stage, and wind direction and intensity. The 

 pressure differentials and resultant water flows con- 

 tribute to the potential natural resource productivity 

 of a basin by facilitating the movement of organisms, 

 nutrients, organic matter, and wastes from one part 

 of the basin to another. For instance, the export 

 of organic detritus and the flushing of wastes and 

 toxins are important to the maintenance of biological 

 production in open water areas. In this context, man- 

 made impoundments or canals modify water flow, 

 thus changing these hydrologic processes. 



Mean salinity and sahnity range at any given site 

 in the basin are detennined by the relative volume, 

 timing, and duration of upstream (fresh) and down- 

 stream (saline) inputs. Sediments and nutrients are 

 distributed among various basin habitats by fresh- 

 water inflows and by currents produced by density 

 gradients (salinity). 



In summary, the hydrologic submodel symbolizes 

 the physiograpltic configuration of a basin that, 

 together with upstream and downstream water 

 sources, determines the water level and water flow 

 regimes and the salinity and turbidity regimes at any 

 point in the basin. These parameters in turn control 

 the type of habitat that can develop at that site and 

 directly influence the productivity of those habitats. 



3.1.2 LAND-MODIFYING PROCESSES 



Over the past several thousand years^ the domi- 

 nant trend in the Chenier Plain has been an increase 

 in wetlands, concurrent with the fonnation of new 

 chenier ridges, at the expense of aquatic habitats. 

 In contrast, during the past 50 years, the major 

 change has been loss of wetlands (fig. 3-1). Subsi- 

 dence and erosion that lead to wetland degradation 

 and its conversion to open water are natural geo- 

 logic processes. But these natural processes have been 

 accelerated by modifications, such as canals and con- 

 trol structures, which have changed basin hydrology. 

 Also, impoundment of weflands for waterfowl, or 

 drainage for agriculture, industry, and urban use 

 result in wetland degradation. These changes may 

 result from activities outside the basin. For example, 

 maintenance of the present Mississippi River course 

 on the eastern side of^ the Mississippi Delta during the 

 20th century has meant that, until the recent growth 

 of the Atchafalaya River, very little new sediment 

 reached the Chenier Plain. Modification of ridge and 

 upland areas is not depicted in figure 3-1 , but changes 

 in these habitats have also occurred through residen- 

 tial and urban development. 



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