MISSISSIPPI EIVER IMPROVEMENT. 



531 



of the Engineer Corps of the Army; Mr. 

 Henry Mitchell, of the Coast and Geodetic 

 Survey ; and three civilians Mr. J. B. Eads, 

 engineer and constructor of the jetties ; Ma- 

 jor B. M. Harrod, Chief -Engineer of the State 

 of Louisiana; and Mr. Benjamin Harrison. 

 The commission, thus ably constituted, made 

 a preliminary report, which was presented to 

 Congress and referred to the Committee on 

 Commerce, March 10, 1880. 

 The work assigned to the commission was : 



1. To direct and complete such surveys of the Mis- 

 sissippi Eiver, between the head of its passes near 

 its mouth and its head-waters, as were then in prog- 

 ress, and to make such additional surveys and exam- 

 inations of said river and its tributaries as might by it 

 be deemed necessary. 



2. To take into consideration and mature such 

 plan or plans as will correct, permanently locate, and 

 deepen the channel, and protect the banks of the Mis- 

 sissippi Kiver ; improve and give safety and ease to 

 the navigation thereof; prevent destructive floods, 

 and promote and facilitate commerce and the postal 

 service ; and, with such plans, to prepare and submit 

 estimates of the cost of executing the work. 



3. To report specifically upon the practicability, fea- 

 sibility, and probable cost of the plans known as the 

 jetty system, the levee system, and the outlet system. 



Finding that accurate surveys of the upper 

 Mississippi had been already completed, the 

 commission directed its attention chiefly to the 

 eleven hundred miles along the channel, from 

 the junction of the Ohio to the head of the 

 passes. Of this part of the river, detached 

 surveys give some available data. Such neces- 

 sary work as was done to connect these was 

 permanently marked, to furnish a basis for 

 future operations. Eor this purpose a line of 

 secondary triangulation was begun, its trian- 

 gles closing within six seconds. 



Physical phenomena were investigated, and 

 the results compared with those of previous 

 researches. Lines of precise level were run. 

 The gauging-stations between Cairo and New 

 Orleans were increased to twenty, which, in 

 addition to those recently established between 

 Cairo and St. Louis, measure the rise and fall 

 of the river at every fifty miles of its course. 

 Thus the progress of floods and the peculiari- 

 ties of the river-slope may be traced. Width, 

 depth, and curvature were noted at different 

 and contrasted points. This survey is being 

 executed, the employees of this commission 

 beginning at Cairo and moving down-stream, 

 while the Geodetic and Coast Survey are as- 

 cending, having begun in the lower river. Mr. 

 Patterson, Superintendent of the Coast Sur- 

 vey, has four triangulating parties in the field. 

 All marks are placed beyond reach of erosion 

 during the next twenty years. The Hydro- 

 graphical and Topographical Corps have estab- 

 lished theirs in pairs, about three miles apart 

 on each bank of the river. The leveling par- 

 ties began their work at Columbus, twenty 

 miles below Cairo, General Cornstock having 

 previously executed a series of levels to that 

 point, the localities selected for these lines 

 were those where similar inquisitions had been 



frequently repeated, in order thus to ascertain 

 the varying relations of the river and its bed. 

 u The observations at these places comprise 

 the changes in the figure of the river-bed at 

 different stages of the water, the kind of mate- 

 rial forming or moving along the bed, the fig- 

 ure and movement of sand-waves, the slope of 

 water-surface, the discharge, and the transverse 

 currents of velocity." Boring-parties were 

 employed in determining the depth of alluvial 

 deposit and the nature of underlying strata in 

 those portions of the river requiring improve- 

 ment. 



Systems of Improvement. After mature de- 

 liberation a majority of the commission passed 

 unfavorably upon the outlet system, as one of 

 diffusion and waste and not of concentration. 

 From irrefragable data and through close rea- 

 soning, they extract these general principles : 



If the normal volume of water in a silt-bearing 

 stream, flowing in an alluvial bed of its own forma- 

 tion, be permanently increased, there will result an 

 increase _ of velocity, and consequently of erosion and 

 silt-bearing power ? an increase in area of average cross- 

 section, and an ultimate lowering of the surface-slope ; 

 and, conversely 2 if the normal flow be decreased in 

 volume, there will ensue a decrease of velocity, silt- 

 transporting power, and mean sectional area, and an 

 ultimate raising of the surface slope. 



The outlet system has been advocated be- 

 cause, during a flood, a crevasse has the effect 

 of lowering the surface of the river in its vi- 

 cinity and below it. Hence, it is argued that 

 natural or artificial outlets, conveying the sur- 

 plus waters to the sea by a shorter route, must 

 tend to prevent the recurrence of disastrous 

 floods. This might be true were it not that 

 the Mississippi holds in suspension a vast pro- 

 portion of silt, borne onward by its mighty 

 current to the Gulf. The velocity of this cur- 

 rent must be maintained ; if checked, a deposit 

 of sediment occurs. Shoals, caused by loss of 

 velocity, are formed below crevasses. If the 

 crevasse is kept open indefinitely, the shoal 

 continues to extend down the stream. Only 

 when the width of the river has been reduced 

 and the current restored does it wash out its 

 channel. 



The Mississippi, not only in its main line but 

 in its outlets, is no exception to the law that 

 silt-bearing streams, flowing through alluvial 

 deposits, have the power to increase or steepen 

 their surface-slopes, so adjusting them as to 

 allow their currents to carry onward the sedi- 

 mentary matter with which they are charged. 

 Other conditions being the same, the slope is 

 least where the volume is greatest, and, con- 

 versely, the slope is increased where the vol- 

 ume is diminished. The following examples 

 are cited : 



The fall of the Atchafalaya is about six inches per 

 mile from its head to the Gulf-level, while the fall of 

 the Mississippi from the same point is less than two 

 inches per mile. The volume of the Atchafalaya _ is 

 only about one twelfth as great as that of the Missis- 

 sippi where they separate. The fall of the South Pass 

 is three inches p^cr mile, while that of the Southwest 

 Pass is but two inches per mile. The volume of the 



