HYDROLOGIC UNIT DESCRIPTIONS 



At the hydrologic unit level of 

 resolution, habitats are the functional 

 components. The primary connections 

 between habitats are water flows with 

 their dissolved and suspended loads of 

 nutrients and organic matter, natural 

 animal migrations, harvests of economi- 

 cally important plants and animals, and 

 flows of urban and agricultural waste 

 products. 



The 20 habitats distinguished in 

 the MDPR can be grouped into three broad 

 categories: (1) high land or terrestrial 

 habitats, (2) intertidal or periodi- 

 cally-flooded habitats, and, (3) aquatic 

 or continuously flooded habitats. Net 

 hydrologic flows through a basin can 

 generally be considered to pass from 

 high land to intertidal to aquatic hab- 

 itats. This means that by-products of 

 the first two habitat groups are often 

 "washed" into aquatic habitats and 

 assimilated by them. This is a matter 

 of great importance in terms of natural 

 ecosystem functions and human perturba- 

 tions of these functions. 



A major feedback flow from the 

 downstream habitats to the socioeconomic 

 system is the harvest of marketable 

 organisms (fishery, game, and fur spe- 

 cies) from the natural coastal ecosys- 

 tem. Also important are a broad range 

 of non-marketed ecosystem services, such 

 as waste assimilation, climate control, 

 marine energy inputs to coastal produc- 

 tion, nursery ground and migratory bird 

 access, aesthetics, and recreation. 



Money flows and purchased goods and 

 services connect urban-industrial habi- 

 tats with the external economy. Agri- 

 cultural and urban habitats produce 

 waste by-products that must be assimi- 

 lated by other habitats in the coastal 

 ecosystem. 



Relative proportions of area cov- 

 ered by the 20 habitats differ greatly 

 from basin to basin. The relative area 

 of a habitat may indicate its importance 

 in the basin in question, especially 

 habitats in Groups 2 and 3. However, 



Group 1, or high land habitats, often 

 show a functional significance greater 

 than their areal extent would indicate, 

 because: (1) urban- industrial , agri- 

 cultural, and other socioeconomic pro- 

 cesses tend to be concentrated in upland 

 habitats and many waste materials are 

 released downstream into flooded zones, 

 and (2) any physical relief or elevation 

 (either natural or manmade) in an 

 otherwise flat coastal area is almost 

 certain to affect water flows and pro- 

 ductivity in flooded areas. 



Specific hydrologic units contain 

 unique distributions of habitats based 

 on their natural and economic histories. 

 For example, the Atchafalaya basin con- 

 tains no salt marsh because it is a new 

 emerging delta with large fresh water 

 inputs. 



In the following sections, each of 

 the seven hydrologic units in the MDPR 

 are described. Some hydrologic units 

 (such as Pontchartrain, Barataria, and 

 Atachaf alaya) have been the subject of 

 much scientific study over the years, 

 while others have received less atten- 

 tion. These differences in both the 

 quantity of information and the types of 

 information available are reflected in 

 the descriptions. For example, the 

 Barataria basin has been the subject of 

 detailed study on eutrophication and 

 other human impacts. These topics are 

 therefore emphasized in the Barataria 

 hydrologic unit description. The 

 Atchafalaya Delta has been the site of 

 much study on sedimentation patterns and 

 marsh colonization, and these topics are 

 emphasized in the Atachafalaya hydro- 

 logic unit description. Some hydrologic 

 units are less well understood and their 

 descriptions are necessarily more gen- 

 eralized. 



MISSISSIPPI SOUND HYDROLOGIC UNIT (I) 



The Mississippi Sound hydrologic 

 unit occupies the eastern flank of the 

 deltaic plain. It is bounded on the 

 north by the 15-ft elevation contour, on 

 the west and southwest by the Pontchar- 

 train hydrologic unit, on the south by 

 the 3 mile limit, and on the east by the 



89 



