hydrologic patterns and the conversion 

 of marsh to open water. 



Spoil banks, the inevitable result 

 of canal dredging, also lead to direct 

 marsh destruction (see the spoil bank 

 section under HABITAT DESCRIPTIONS) . 

 Spoil is usually deposited alongside the 

 dredged area, a practice that destroys 

 the adjacent marsh and results in a bar- 

 rier to water exchange with the adjacent 

 marsh. The ratio of canal to spoil area 

 has been estimated to be 1:2.5 (Craig et 

 al. 1979), indicating the importance of 

 spoil deposition to wetland loss. 



Canals also have less obvious in- 

 direct effects due to such factors as 

 saltwater intrusion, altered sediment 

 transport, and conversions of streamside 

 to inland marsh. A subtle yet pervasive 

 indirect effect of canals is a decrease 

 in sedimentation that leads to a decline 

 in marsh elevation. When a canal is 

 dredged through a marsh, the canal 

 replaces the natural channels (tidal 

 streams) as the main route of water 

 transport through the marsh, because 

 canals are deeper and faster flowing 

 (Craig et al. 1979). The streamside 

 marsh adjacent to the natural channel is 

 steadily converted to inland marsh, 

 because the natural channel no longer 

 supplies the sediments and nutrients 

 necessary to maintain it. Once con- 

 verted to the inland type, the marsh is 

 then subjected to the sediment deficit 

 described earlier, and will eventually 

 be converted to open water because of 

 subsidence. The estimated area of 

 streamside marsh converted to inland 

 marsh by the indirect mechanism is three 

 to four times the area of streamside 

 marsh converted directly to open water 

 by canal building (Craig et al . 1979). 



Several recent studies have looked 

 at the statistical relationship between 

 canals and wetland loss, by examining 

 different parts of the Louisiana coastal 

 zone and comparing their recent rates of 

 wetland loss as a function of canal 

 density. Scaife et al. (1982), and 

 Turner et al. (1982) performed linear 

 regression analyses of several variables 

 on wetland loss rates. Data for these 



studies were canal densities on indi- 

 vidual quadrangle sheets of the MDPR as 

 shown in USFWS habitat maps compiled by 

 Wicker et al. (1980b). These studies 

 concluded that canals in the MDPR have 

 been responsible (directly and indi- 

 rectly) for as much as 65% or more of 

 the total wetland loss between 1955 and 

 1978, and that the area of new canals 

 added each year is accelerating in the 

 MDPR. 



Deegan et al. (1982) followed up on 

 these studies, performing multiple 

 regression analyses on the same data, 

 and are somewhat more conservative in 

 their assessment of the relationship 

 between canal density and wetland loss. 

 Nevertheless, the authors of the latter 

 study concluded that from 33% to 67% of 

 the loss of wetlands during the 23 year 

 study period could be a result of 

 canals. A total of 58% of the variation 

 in wetland loss among 140 quadrangle 

 maps was explained by a multiple regres- 

 sion model that included canal densities 

 at each time, relative geologic age of 

 the delta lobe in which the area was 

 located, and the specific hydrologic 

 regime of the area. By comparing the 

 amount of land that had been lost due to 

 the increase in canal area (Deegan et 

 al. 1982) concluded that up to 20% of 

 the wetland loss could be accounted for 

 as a direct conversion to canal area. 



Another recent study that examined 

 the effects of canals on wetland loss 

 used a different approach, that of a 

 simulation model of land change pro- 

 cesses in the Barataria basin (Cleveland 

 et al. 1982). This model showed a 

 marked acceleration of both streamside 

 and inland marsh loss when canals were 

 included in the simulation. 



EUTROPHICATION 



Eutrophication is the natural or 

 artificial addition of nutrients to 

 water bodies, and the effects of these 

 added nutrients on the water body. Al- 

 though eutrophication is a natural 

 process, it has been accelerated in many 

 cases by human activities. Severe 



154 



