modifications lend themselves to evaluation. In the 

 Chenier Plain, the management of water in the entire 

 Mermentau Basin, the navigation channel in the Cal- 

 casieu Basin, the upstream reservoir and major ship 

 channel in the Sabine Basin, and the Gulf Intracoastal 

 Waterway (GIWW) are examples. All except the GIWW 

 example are discussed in part 3.6. The GIWW runs 

 from east to west across all of the basins except the 

 Chenier Basin. In spite of its length and importance, 

 there appears to be no comprehensive quantitative 

 study of the hydrologic impact of the GIWW. It is 

 known to facOitate the flow of water laterally across 

 basin boundaries; an average flow of 110 m^/sec 

 (3,885 ft^/sec) occurs westward from the Sabine Basin 

 to the East Bay Basin. The quality of this water de- 

 pends on its proximity to channels interconnecting 

 with the Gulf; the GIWW can carry saline waters into 

 formerly freshwater areas. In addition to facilitating 

 east to west water flow, spoil banks of the GIWW are 

 significant barriers to overland sheet flow and may 

 also disrupt local animal movement patterns. North 

 and south portions of the basins are, in effect, hy- 

 drologically cut off from each other by the GIWW. 

 As a result, sahnity and vegetation gradients across 

 the GIWW can be much sharper than elsewhere in the 

 basins. 



The GIWW and other major pubhc works are 

 superimposed on a history of many smaller activities 

 that have modified hydrology during the past 100 

 years or more. Their cumulative impact has been ex- 

 tremely difficult to evaluate because the effects have 

 occurred over a long period of time. There is at least 

 minimal infomiation on freshwater flows from up- 

 stream gaging stations as well as some data on saUnity. 

 However, data on sediment inputs are extremely 

 scarce. Net sediment deposition and erosion rates can 

 be deduced from maps and aerial imagery. However, 

 without a large major modehng effort, the ability to 

 detect and/or predict modifications of water levels, 

 circulation patterns, and wetland flooding regimes is 

 limited, and only large-scale effects can be documented 

 with any confidence. 



The hydrologic effects of canals and their associ- 

 ated spoil banks are difficult to document quantita- 

 tively. Canals are a major measureable feature of hu- 

 man occupancy of the Chenier Plain, and there are 

 8,714 km (5,415 mi) of canals of various types (table 

 3.52). Plates 5 A and 5B display the distribution of 

 these canals. About one-third of the total are agricul- 

 tural drainage canals; additional canals were con- 

 structed for other purposes and only incidentally 



Table 3.5 1. Flow model of primary' and secondary effects of cultural modifications of the hydrologic 

 regime on the Chenier Plain ecosystem. 



Secondary effects on 



I habitat type and area 



Part 3.4.3 



Ueposition/erosion/subsidcncc 



Salinity shifts 



Impoundments 



Spoil areas 



Drained areas 



Habitat interaction-interface effects 



Eutrophication (part 3.5.3) 



Secondary effects on 



habitat function 



Part 4.0 



Community structure 

 Productivity 

 Community distribution 

 Flux of organic materials 

 Flux of inorganic nutrients 



Secondary effects 

 on biota 

 Part 5.0 



Migrations-routes and timing 



Salinity tolerance 



Food availability 



Substrate suitability 



Production 



Access 



72 



