604 



SCIENCE. 



[N. S. Vol. XIX. No. 485. 



it, with all the use that could be made of 

 existing sti-eams and bayous, would require 

 the removal of earth enough to build sev- 

 eral times its length of levee. Neverthe- 

 less, with men and money and time enough 

 it could be done. And if it were con- 

 structed, and in order at the oncoming of 

 a great flood, I should suppose that for 

 once it would materially lower the high- 

 water elevation except in those parts of the 

 river in the neighborhood of the crossings 

 of the main river by the subsidiary channel. 

 So far up and down stream as that engorge- 

 ment extended it would be necessary to 

 maintain levees high enough to take care 

 of the water, to whatever stage it might go. 



It would be the extreme of folly, how- 

 ever, to construct so costly a work without 

 the assurance of its permanent utility. The 

 probability of this result may be considered 

 from two hypothetical points of view. 

 From the first of them would be contem- 

 plated, I may say, a subsidiary channel 

 sufficient in capacity to carry only a small 

 part of an extreme flood — just the two or 

 three feet on top which produce the greatest 

 strain and danger under present conditions. 

 Such an outlet opening would have to be 

 very carefully constructed and guarded, in 

 order to prevent its indefinite enlargement ; 

 but that could be done. For that purpose 

 it would be desirable to take the water off 

 through a number of small openings lead- 

 ing to the subsidiary channel as indicated 

 on the map. 



The extreme floods which this provision 

 would be designed to relieve occur only 

 rarely — not oftener, upon an average, than 

 once in five years. During the intervals 

 the unused channel, especially those parts 

 of it not following the channel of some 

 river or bayou, would be filled with a dense 

 growth of vegetation— willows and cotton- 

 woods, mostly. The fioods have no regular 

 periods of return, so that there would be no 

 way to be sure that the channel would be 



iinobstrueted except to keep it open and 

 clean all the time. This could be done too, 

 but it would involve an annual expense 

 equal to the cost of clearing a right of way 

 for a railroad from Cairo to the gulf. It 

 would be expected, I suppose, to lay out 

 the subsidiary channel on comparatively 

 straight lines down the interiors of the 

 basins. This would result in a high ve- 

 locity of flow in them ; and this, again, 

 would result in more or less cutting and 

 caving of banks, with the consequent forma- 

 tion of bars. There would be danger that 

 the subsidiary channel would follow the 

 vicious example of its parent and overflow 

 its banks; and to be secure against this 

 danger it would probably be necessary to 

 restrain its inherited propensities by levees. 



It follows that, even with this smallest 

 subsidiary channel that would suffice to re- 

 lieve the tension of an extreme flood, we 

 would have on our hands a work of pro- 

 digioiTS magnitude and cost which, at its 

 best, would relieve us of only a small part 

 of our present burdens and dangers. It 

 would be necessary to continue to maintain 

 the levees on the main stream. They would 

 be subject to the same accidents which be- 

 fall them now. Caving banks would un- 

 dermine them, and muskrats would burrow 

 in them as now. Considerations of expense 

 would require us to build on as low grades 

 as would be consistent with safety, just as 

 we do now. When a great flood came there 

 would be the same apprehension of disaster, 

 the same necessity for incessant watch- 

 fulness, and the same occasional crevasse 

 which attend the floods now. Assuming 

 that our outlets and subsidiary channel 

 were entirely successful in accomplishing 

 the work for which they were designed, we 

 would still be little better off than we are 

 now with the added burden of the enor- 

 mous cost of the subsidiary sj'stem to be 

 carried forever. 



This, it is to be remembered, is upon the 



