SCOTT IMPROVEMENT OF THE WESTERN RIVERS. 49 



theory, while the work done at Cahokia Bend and Horse-tail in- 

 dicate a change in their plans. 



This system is well adapted to clear streams flowing over a 

 rock bottom and having a good fall and rapid current ; but when 

 the fall is hardly sufficient to carry the sediment, it is destructive, 

 and the more so if the bottom and sides are alluvial, as it causes 

 a more rapid deposit of the sediment by slackening of the current, 

 and gradually destroys the navigation it was designed to improve. 

 By forcing the current against the alluvial banks the curves be- 

 come deeper, and the diminishing velocity, together with the con- 

 sequent widening, results in the formation of a shoal. 



It will be of use to examine the cost of their work when com- 

 pleted, and, if possible, ascertain before we spend much more, 

 whether anything can be done by the present system that will 

 pay for the outlay. 



We may take the piers of the St. Louis and Illinois bridge for 

 the depth, and estimate from the surface of low water to bed-rock 

 as 100 feet, premising that a dam or dyke built of broken stone 

 (as has been used heretofore) will sink until it reaches the bed- 

 rock, imless built above the highest floods. 



This dyke must extend at least lo feet above low water when 

 finished, with a surface of 5 feet on top, with sides of not less than 

 45 degrees slope ; this gives us a sectional area of 12,650 feet or 

 2,024,000 perches per mile, costing over one dollar per perch. It 

 would require over thirty miles of dyke to secure the river from 

 St. Louis to Cairo, and when we get below Cairo it is safe to 

 double the depth and quadruple the cost. It will require but little 

 knowledge of the public to know that such expense cannot be 

 borne by it. 



It is true that the engineer in charge (Col. Simpson) has en- 

 deavored to find a less expensive method, but the same agency 

 that wears the solid rock below a fall will destroy any solid dyke 

 built on the quicksands of the Mississippi unless it extends above 

 the highest water ; that agency is the reacting eddy washing out 

 the sand below if at right angles, and above if diagonal, to the 

 current, causing it to sink and break up in a short time. 



The dyke across Alton Slough is a fair test of this kind of work 

 in comparatively slack water and out of the main current alto- 

 gether, but the test now being made in Cahokia Bend will prove 



