Apkil 15, 1904.] 



SCIENCE. 



605' 



assumption that the outlets would be de- 

 signed to take from the main channel only 

 the surplus of an extreme flood over the dis- 

 charge of an ordinary flood. To consider 

 the subject from the other point of view 

 referred to, we may suppose the system of 

 outlets and channels already described to 

 be made of sufficient capacity to carry all 

 the surplus water above the overflow stage, 

 so that the levees on the main stream could 

 be abandoned. This would require chan- 

 nels of far greater size and cost. But as 

 the plan would propose to dispense with all 

 levees and so save the cost of them, we may 

 set off that saving against the cost for the 

 present purpose, and confine our attention 

 to questions of maintenance and effective- 

 ness. 



In such a system the subsidiary channel 

 would be only another river. In all floods 

 the two channels would divide the discharge 

 between them, and water would flow in both 

 of them all or a large part of the time. 

 Would the river be able to maintain for 

 itself as ample discharge room in the ag- 

 gregate by a divided flow through two par- 

 allel channels as by a concentrated flow 

 through a single channel? To state the 

 question is to ansM'er it. The smaller the 

 channel by which a fluid flows the greater, 

 relatively, is the retardation due to friction. 

 A river flows with greater velocity at high 

 stages than at low stages because of its 

 greater volume. A flood divided between 

 two channels would have less power to scour 

 out and keep open the two channels than it 

 would have to scour out and keep open a 

 single channel. The two channels would 

 have a greater tendency to fill up by de- 

 posit of sediment than a single channel 

 carrying the whole discharge would have. 

 There can be no dispute over these proposi- 

 tions among engineers. 



It is by reason of the immutable opera- 

 tion of these laws that the Mississippi River 

 has made for itself a single great channel 



from Cairo to the sea. As between two 

 parallel streams produced by division the 

 smaller stream is the weaker. As it shrinks 

 in capacity by deposit, what it loses in vol- 

 ume of discharge the other stream gains. 

 Thus the disparity between them in voliune 

 and energy increases at an increasing rate 

 until the smaller channel is obliterated and 

 the larger stream takes the whole discharge. 

 To attempt to fight that tendency toward 

 concentration in so great a river as the 

 Mississippi flowing through a material so 

 easy to erode and so ready to sink would 

 be a futile undertaking. 



I have thus discussed two imaginable out- 

 let schemes — one a mere tapping, or blood- 

 letting, operation to take off the top layer 

 of an extreme flood, leaving the levees to 

 take care of all the lesser floods; the other 

 a true subdivision of flow complete enough 

 to obviate the necessity of levees by pro- 

 viding sufficient channel capacity to carry 

 all floods without overflow. It requires, as 

 it appears to me, only a little close atten- 

 tion to the subject to make it apparent that 

 they are both hopelessly impracticable. 



In the consideration of the latter of the 

 two plans stated — that one assuming a gen- 

 eral abandonment of levees, and a reliance 

 upon outlets and subsidiary channels as sole 

 protection against floods, I have not taken 

 into account the problems which would be 

 presented at the intersections of the sub- 

 sidiary channels and the main channel, be- 

 cause the argument seemed to me to be 

 sufficient without considering them. I 

 think, also, that it would be sufficient with- 

 out considering anything but them. 



Such an outlet scheme as I have supposed 

 is the fairest one I can think of for illus- 

 tration. It is not physically impossible. 

 The soil of the alluvial valley can be fash- 

 ioned in any shape you choose. There is 

 an imaginable sum of money which would 

 do the work. It would be a less violent 

 contradiction of the natural course of 



