below ground. Canals abound in every parish, in every wetland plant community and soil 

 type, and have increased gradually, not suddenly, in density. In effect, a giant 

 experiment is being conducted and we have only to recognize it as such to evaluate the 

 results. The random surface distribution of canals and their differences in density over a 

 wide geographical area and in different geological surface substrates provides a 

 laboratory for the examinaton of their effects on a variety of wetland processes. This 

 study represents some preliminary assessments of the relationship between land loss and 

 canals, based on recently acquired, detailed area measurements. 



The Louisiana coastal zone has grown seaward for 7,000 years at a new steady gain 

 of 500 to 600 ha annually. Since 1900, however, there has been a net annual loss of 

 land. The annual land loss rates have increased as the number of canals has increased. 

 The prevalent explanation for the cause of the acceleration in land loss rates usually 

 relies heavily on two arguments: first, that the disruptive influence of the Mississippi 

 River levees reduces natural overbank flooding and shunts sediments offshore, and 

 second, that there is a natural decay of deltas. Canals are generally considered ancillary 

 factors in this explanation (e.g., Gagliano et al. 1981). There is a qualitative 

 attractiveness to this argument, with which one of us has grappled before (Craig et al. 

 1980), but the data for a quantitative evaluation were limited then. Now, however, we 

 have new data (Wicker 1980) to support the examination of an alternative hypothesis: 

 that canal density is directly correlated with increased land loss rates at the local and 

 regional levels and through time, and that impact of canals varies with changes in soil 

 conditions and proximity to sediment sources. It is worth mentioning at the outset that 

 the point of this exercise is not to place blame on one factor or another but, instead, to 

 help understand what is happening and, thus, help provide for the enlightened and 

 effective management of these valuable renewable resources. 



CANAL DENSITY 



Major inventories of canals and land loss in the Louisiana coastal zone have been 

 conducted by Barrett (1970), Gagliano et al. (1971, 1981), Chabreck (1972), Gosselink et 

 al. (1979), and Wicker (1980). These are the sources we will use in the figures that 

 follow. The different surveys have various geographic boundaries that may not 

 coincide. The most extensive data set available is for the deltaic plain, which extends 

 from the Mississippi-Louisiana border to just west of the emerging Atchafalaya delta. 

 We have normalized inconsistencies in geographic boundaries by expressing the area of 

 canals as a percentage of annual loss based on the change from the initial to the later 

 conditions. 



The average canal density for the whole deltaic plain has increased steadily since 

 1890, when we presume there were very few canals (Figure I). The canal area has 

 climbed geometrically with time. From 1955 to 1978, it increased from 1% to 2.4%, or 

 at a doubling rate of around 20 years. Including spoil banks, the total land area affected 

 approached 10% by 1978, a magnitude equal to the surface area one would expect the 

 natural drainage features to occupy in an unaltered marsh. The relationship between 

 natural channel density and canal surface area is an inverse one (Craig et al. 1980). 

 Natural channel density decreased logarithmically, while canal density increased linearly 

 in the vicinity of the Leeville oilfield. The natural hydrology is obviously altered by the 

 reduction in lateral flooding as a result of the spoil bank levees, by obstructing natural 

 channels, and by the linear and uniform conduit created by the canals. 



74 



