34 



TREADWELL 



NAVIGABILITY OF WATERWAYS 



In order to travel in most of the area, one must rely on boats; therefore, an understand- 

 ing of water- depth distribution is very important. 



Water deep enough for an ocean-going vessel is present to within one to five miles of the 

 Chandeleur Islands on the Gulf side. Chandeleur and Breton Sounds and Lakes Borgne and 

 Pontchartrain are between five and twenty feet deep. The more shallow parts lie close to 

 shore. Except for the large water bodies just mentioned, marshland lakes are nearly always 

 less than six feet deep. Deeper water is confined to "passes" between lakes. Depths of lakes 

 can generally be correlated with size— the larger, the deeper. 



Tidal streams of this area vary in depth from less than a foot to a hundred feet in some 

 cases, such as the channel between Lake Pontchartrain and Mississippi Sound. The width of a 

 stream is a reflection of its depth; however, allowances must be made for its origin. Although 

 origins are diverse, only two types need be considered: tidal streams originating between 

 levees of former distributaries and tidal streams forming in "open" or imobstructed marsh. 



Tidal streams develop between natural levees by adopting the remnant distributary course, 

 producing rather straight, frequently long, channels. This pattern is evidenced even after five 

 or ten feet of levee subsidence. However, streams that develop in marsh with no obstructions 

 meander considerably and rarely have the straight pattern associated with tidal streams 

 confined by levees. 



The width and greatest depth of ninety tidal streams are plotted on Fig. 4. The chart is 

 divided into three bands. The uppermost represents the depth-width relationships of tidal 

 streams originating between levees; the central band represents the depth-width relationships 

 of tidal streams forming in open marsh; the lower band shows that bends of tidal streams, like 

 bends of gravity- motivated streams, are deeper than reaches. By measuring the width of a 

 stream on an aerial photograph or map, it is possible to predict the approximate depth of that 

 stream. Furthermore, the chart shows that streams' originating in open marsh are deeper for 

 a given width than those formed between natural levees. Although a few exceptions to the limits 

 presented are known, the chart may be used with success at face value. However, consideration 

 of the size and shape of a stream's drainage basins allows even closer depth prediction. The 

 chart was constructed from data collected in easternmost Louisiana, but it has been found appli- 

 cable throughout the marshlands of the State. This is an area of fine sediment and extremely 

 low tidal range (1.5 feet). The effect of greater tides and coarser sediment on the depth -width 

 relationship of tidal streams will be studied this summer in the New England coastal marshes. 

 Necessarily, a new set of values will have to be compiled for this region. 



SEDIMENTATION 



Figure 4. Depth- width relationship 

 of tidal streams. 



In addition to work on trafficability and naviga- 

 bility, a hundred and twenty sediment samples were 

 taken and analyzed for grain size, sorting, organic 

 content, percent shell, and glauconite and pyrite 

 content. 



It was found that "clean" sands are present near 

 the Chandeleur Islands. Muddy sands or silts bottom 

 the sounds, and the lake beds are composed of silty 

 clay and clayey silt. 



ECOLOGY 



From the sediment samples about sixty species 

 of Foraminifera and a similar niunber of species of 

 MoUusca have been identified and their ecologic rela- 

 tionships determined. 



