532 



MISSISSIPPI EIVER IMPROVEMENT. 



South Pass is only about one quarter as large as that 

 of the Southwest Pass. 



As water selects the line of least resistance in flow- 

 ing from a higher to a lower level, it follows that, in- 

 asmuch as that portion of the Mississippi floods which 

 enters the Atchafalaya seeks the Gulf-level through a 

 route not half so long as that which follows the main 

 river, the resistance in the shorter and steeper route of 

 the Atchafalaya must be so much greater, that these 

 elements which tend to increase the current are so far 

 neutralized as to produce in both routes to the sea that 

 rate of current which is capable of transporting the 

 sediment without loss or gain to the Gulf-level, and 

 thus a condition of equilibrium is established between 

 these two routes to tne sea. It seems unnecessary to 

 state that the ratio of frictional resistance to volume of 

 water j resulting from the smaller size of the Atchafa- 

 laya, is so much greater than that in the main river 

 that this condition of equilibrium or regimen of the 

 two channels is the result. Anything which will 

 tend to increase the flow permanently through either 

 route would, if unchecked, have a tendency to cause 

 the entire river to find its way ultimately through that 

 route to the sea, by lessening in it, as it enlarged, the 

 ratio of frictional resistance to volume of water flowing 

 in it. The sub-delta building ability of the smaller 

 passes, by _ which they prolong their length and thus 

 flatten their slopes, will invariably tend to cause their 

 extinction, by results similar to those, hereinafter re- 

 fered to, at Cubitt's Gap, The Jump, and the extinct 

 outlets below them. This cause has tended to the 

 extinction of many well-known bayous below the Atch- 

 afalaya. That the Atchafalaya remained so long un- 

 unaltered, and is now evidently enlarging, is owing to 

 important changes in the bed of the Mississippi near 

 it, by which a large portion of the floods of Ked River 

 have been recently discharged through it. 



This explanation of the relation between slope and 

 volume is, of course, applicable to the other existing 

 outlets referred to in this connection. For tnis reason 

 the commission believes that no surer method of ulti- 

 mately raising the flood-surface of the river can be 

 adopted than 'by making lateral outlets for the escape 

 of its flood-waters. The raisiug of the flood-surface 

 necessitates an increase in the neight of the levees, 

 and leaves shallower channels for navigation. 



As the system of improvement proposed by the com- 

 mission is based upon a conservation of the flood- 

 waters of the river, and their concentration into one 

 channel of an approximately uniform width, it would 

 seem scarcely necessary further to consider a system 

 based upon theories and arguments so diametrically 

 opposed to it as the outlet system is thus shown to be. 



An outlet, ten miles below New Orleans, 

 into Lake Borgne, was proposed by Engineer 

 Ellet in 1852, in a report to the Secretary of 

 War. His plan was examined and rejected by 

 Humphreys and Abbot, and again in 1875 by 

 the Levee Commission. It is an erroneous 

 idea to suppose that such an outlet would be 

 permanent. Similar openings in the river 

 show that a sub-delta formation immediately 

 begins, which in a few years effects a closure. 

 Several extinct channels are found on either 

 side of tlie great passes. The largest and most 

 recent of these, known as Cubitt's Gap, oc- 

 curred about seventeen years ago, caused by a 

 narrow canal. The river and gulf were then 

 about a thousand yards apart, with a mean 

 difference of three feet between their surface- 

 heights. This fall gave such rapidity to the 

 escaping flood that it soon made a crevasse over 

 two thousand feet wide and at least one hun- 

 dred feet deep where the river-bank once rested. 

 Once through the gap, the velocity of the water 



was checked, and the sediment deposited over 

 an area of possibly thirty square miles, forming 

 innumerable islands and shoals. The breach 

 in the bank is rapidly closing, and must shortly 

 disappear by a natural process. 



The same result has followed at The Jump, 

 at the great Bonnet Carre Crevasse, and at all 

 openings, natural or artificial, made through 

 the bank. Hence the commission argues that 

 the outlet into Lake Borgne could only be kept 

 open by continued dredging, while a shoal 

 would certainly form below, to the manifest 

 injury of navigation. The slope of the river 

 being increased from the head of the passes to 

 the outlet, the flood-surface above that opening 

 would be raised. 



In regard to the Atchafalaya outlet, ex- 

 cept such work as is requisite to prevent its 

 enlargement, the commission makes no recom- 

 mendation, as Major Benyaurd, United States 

 Engineer Corps, who is in charge of the 

 Government work on that part of the river, is 

 preparing a special report concerning the 

 mouth of Red River. The proposition to turn 

 the waters of the Red River into the Calcasieu 

 is impracticable, the bed of the Calcasieu being 

 from sixty-three to seventy-three feet higher 

 than the water-surface of the Red River at the 

 closest connectable points, which are 23'86 

 miles apart. If it is proposed to make the 

 connection near the mouth of the Calcasieu, 

 the expense is prohibitory. 



The effect of crevasses and other outlets in 

 lowering floods has been exaggerated. In 1858, 

 when the Bell Crevasse was pouring through 

 the break at the rate of 80,000 cubic feet per 

 second (about one twelfth of the total discharge 

 of the river at the time), the water-surface was 

 only lowered one and five tenths' foot at the 

 site ; while at Baton Rouge, one hundred and 

 twenty-four miles above, no sensible effect was 

 produced. 



The influence of the Gulf on the river is not 

 felt during high water so far as Red River. 

 The Raccourci and Red River cut-offs, which 

 shortened the course of the river thirty-nine 

 miles and lowered the high-water mark near 

 the mouth of Red River four and six tenths 

 feet in the flood of 1851, produced no effect 

 one hundred miles above. Therefore, both Gulf 

 tides and crevasses are restained in their action 

 to a limited area, and this influence decreases 

 rapidly in ascending. 



The Levee System. Though levees have been 

 built solely to protect alluvial lands from over- 

 flow, they have a direct action on the river 

 itself, deepening its channel and enlarging its 

 bed during seasons of flood, restraining its dis- 

 persion and causing its attainment of a higher 

 level within its banks. During the period be- 

 tween 1850 and 1858, when the levees were 

 kept in best order, the channel of the river 

 was in better navigable condition than it has 

 been since that time. During the last twenty 

 years the levee system has been interrupted by 

 numerous crevasses between Cairo and Red 



