88 WISCOKSIN" ACADEMY SCIENCES, AETS, AND LETTERS. 



jettee'. In fact it appears that at one time the Delta was being 

 forced to the east, as may be observed at Bird Island, where the 

 motion must at some time have been decidedly east. As soon, 

 however, as the delta had advanced far enough to stop the erasion 

 of the shores, the accretion ceased, and the shoals produced by the 

 previous easterly motion gave a tendency to a contrary motion. 

 During freshets the alluvion is precipitated on the banks which 

 thereby continue to rise and to assume a more substantial consist- 

 ance. 



In building a dyke, or causeway of earth, the same is self-snstain- 

 ing to a considerable height; even in shallow water the same can be 

 formed, but when the saturated portion becomes considerable, ifc 

 finally loses the cohesion necessary to support the superincumbent 

 weight and partakes of a lateral motion, or in other words, spreads 

 out, until the submerged portion attains sufficient resistance to pro- 

 duce an equilibrium. 



The alluvion of the delta after reaching deep water, and not being 

 fortified by any material denser than its own impalpable mud, must 

 necessarily spread out until the lateral resistance would prevent mo- 

 tion. The river also would become wider, and lose in depth what 

 gains in extent. Now, considering the foregoing facts, it is 

 not at all surprising that the delta of the Mississippi encroaches on 

 the gulf and presents the difficulties with which we are already fa- 

 miliar. 



The estimated discharge of alluvion of the South Pass is about 

 22,000,000 cubic yards per annum, and the advance of the delta is 

 put at 100 feet. 



As the delta advances, its progress will decrease in proportion as 

 the depth of the gulf increases; the difficulties of navigation 

 would increase in the same ratio in this wide-spread bed of alluvion. 

 In order to estimate the result of this progressive motion, we must 

 consider that the discharge of a stream is the product of two 

 quantities, viz., the cross-section and mean velocity, (Q=F.Vo) and 

 that the latter depends principally upon the absolute slope; 



hence the lengthening of the river would diminish the slope, and 

 since the natural supply must of necessity be discharged, the slope 

 must of necessity adjust itself for the performance of the work. 



