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1975 



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Figure 1. Canal density in the deltaic plain as determined from various 

 surveys (data from Chabreck 1972; Gagliano 1973; and Wicker 1980). 



CANAL DENSITY AND LAND LOSS RATES 



The whole coast is not uniform with respect to canal density and land loss rates. 

 Land loss rates for 1955 to 1978 were as low a -2% annually (a net gain of land) in the 

 active Mississippi River delta and in the Atchafalaya delta. Canal densities vary among 

 the hydrologic units as well (Table I). Some are above 3% and others are below 1%. 

 Canal densities have increased in the last 25 years in every hydrologic unit. There is a 

 general relationship between canal density and land loss rates in each hydrologic unit 

 (Figure 2). The point at which canal density is zero is also where land loss rates are 

 slightly below zero (a net gain). Further, if one looks at the historical changes in land 

 loss rates for the whole region, the same pattern emerges (Figure 3). Land loss is high 

 when canal densities are high. Both were low at the turn of the century and have 

 increased coincidentally since. The first estimates of land loss, for 1891 to the 1930s, 

 are perhaps too high, since the early maps did not delineate marsh ponds and drainage 

 channels. The present land loss rates are considerably more accurate and average about 

 0.8% annually from 1955 to 1978. Now (1982), land loss rates ore near 1% annually. This 

 translates to a regional "half-life" of 50 years. There is no indication that trends in 

 either canal density or land loss rates are changing in Louisiana. 



These latter relotonships were sufficiently interesting to justify comparing land 

 loss rates with canal densities in individual quadrangle sheets of the coastal zone for 

 1955-78 (Scaife et al. in press). Subsidence rates and the substrate in each delta lobe 

 differ (Morgan 1963; Adams et al. 1976). One net effect of delta building is the 

 progradation of younger sediments over older sediments. The latter are more 

 consolidated and therefore more resistant to erosion. Also, wave attack and 



75 



