86 



SCIENCE. 



[Vol. XII. No. 290 



from ^^ to ^TjVif, with an average of ^U part of the volume. Di- 

 rect measurements on the turbidity of the Ohio do not appear to 

 have been made ; but observations on the Mississippi at Columbus, 

 Ky., indicate that the Ohio, like the upper Mississippi, is compara- 

 tively free from sediment. The observations just mentioned show, 

 in fact, that the turbidity of the Mississippi at Columbus, Ky., fol- 

 lows closely the turbidity of the Missouri at St. Charles ; and it is 

 estimated that more than eighty per cent of the sediment in the 

 Mississippi at Columbus comes from the Missouri. The amount 

 of sediment carried into the Gulf is less certain than the amount 

 poured into the valley at Cairo, since the load brought in by the 

 minor tributaries and the load carried off by the Atchafalaya are 

 unknown. But under any reasonable supposition concerning the 

 carrying capacity of the Atchafalaya, it appears that from -^^^ to 

 53^ of the total discharge into the Gulf is mud ; and, on compar- 

 ing these figures with the corresponding values for the Missouri, it 

 appears that this tributary furnishes from forty to sixty per cent of 

 that mud. 



It is seen, then, that the Mississippi from its junction with the 

 Missouri bears onward to the Gulf a load which increases with the 

 accession of every affluent. But the bald figures cited do not 

 readily give an adequate impression of this important fact. Let it 

 >be stated, then, in another form, and in numbers more readily 

 igrasped. It will suffice to give the output of the Missouri, which 

 Ihas been carefully measured. 



The average discharge of sediment from the Missouri is, in round 

 numbers, 170 cubic feet per second, or 500,000 cubic yards per day, 

 or 180,000,000 cubic yards per year. At flood-stage the discharge 

 of sediment has been observed to be as great as 4,000,000 cubic 

 yards per day. The latter amount is equivalent in volume to a 

 levee 100 square yards in cross-section and 23 miles long, and the 

 average annual output would suffice to build more than 1,000 miles 

 of such levee. The volume poured into the Gulf is about twice 

 this output. 



Now, what is the effect of this sedimentary load on the course 

 and character of the river from St. Louis to the Gulf ? Observa- 

 tions on river-systems, and studies of river-action in general, lead 

 to the recognition of this principle ; namely, that the cutting power 

 of a stream increases rapidly with an increase of sedimentary load. 

 A stream with a clear supply cleans and maintains a fi.\ed chan- 

 nel. Gorge a stream with sediment, and its equilibrium becomes 

 unstable. It cuts away its banks here and piles up sediment 

 there, so that the position of the channel is ephemeral ; and during 

 flood-stage the burden of water is unloaded upon the adjacent 

 lands. That this may be clearly understood, let it be stated in 

 another way. 



When a river receives from a tributary a disproportionately great 

 load of sediment, such sediment is soon deposited, and the channel 

 is thereby choked. This choking is of a peculiar nature ; for the 

 sediment is not deposited evenly along the bottom of the channel, 

 but is thrown down in the quiet waters, that is, it is deposited irregu- 

 larly along the course of the stream, now on one side and now on 

 the other. These irregular deposits turn the current of the stream 

 and throw it against the banks, now on one side and now on the 

 other. By this agency the banks are cut, and the waters of the 

 river are again loaded with sediment, which is again thrown down^ 

 and again the stream is turned against its banks and again loaded, 

 and again deposits are made. It is thus that the original overload 

 of sediment is made the occasion for a series of operations, each one 

 of which serves to choke the channel in such a manner that the 

 floods are thrown out upon the adjacent land. As long as a 

 stream running through a flood-plain is overloaded with sediment, 

 Just so long will it choke its channel, and just so long will it change 

 the position of its channel, and just so long will it inundate the ad- 

 jacent lands of the flood-plain at the time of flood. 



The action of the Mississippi exemplifies this principle on a grand 

 scale. To appreciate its importance, it is only necessary to con- 

 sider the tortuous and constantly shifting course of the reach from 

 the Ohio to the Atchafalaya, and the menacing dangers to deep- 

 water navigation along the lower reach . The upper of these reaches 

 is the region of greatest lateral corrasion or bank-cutting. It is 

 ■here that the abrading materials of the principal tributaries are 

 brought together ; and, impelled by the force of an appropriate 



declivity, they here do their heaviest work. Here they are ground 

 and reground, and dug up and redeposited. Much of the coarser 

 sediment is left, especially during floods, to add to the geological 

 growth of the region, while vast quantities pass on to the sea. The 

 lower reach is at present one of greater stability. The absence of 

 large tributaries, and the escape of floods into the upper basins, 

 give it a steadier flow ; and the mud with which it is loaded is more 

 finely comminuted, and hence more easily transported to the Gulf. 

 But the disturbing element is present, and liable at any flood-stage 

 to work disastrous effects. 



It must be clearly understood that the diminution of the volume 

 of water in the lower Mississippi is not the prime end to be sought. 

 The prime end to be sought, in order to prevent destructive floods, 

 is to prevent the choking of the channel. The storing of flood- 

 waters on the Ohio and on the upper Mississippi would at first re- 

 lieve the lower flood-plain ; but, on the other hand, the choking of 

 the lower channel would afterward progress at an increased rate, 

 and ultimately the storage of such waters would augment the dan- 

 ger and destruction. But the storage of the waters of the Missouri, 

 and other tributaries that are surcharged with sediment, so as to 

 deposit this sediment on the plains, would permit the purer waters 

 to open a sufficient channel for themselves, and the Mississippi 

 plain would thus be protected. The real problem is to relieve the 

 river of its excess of sediment, and thus prevent lateral cutting and 

 promote vertical scouring, and thus provide adequate channel-room 

 for the greatest floods. 



Of the three rivers that contribute the principal volume of flood- 

 waters to the lower Mississippi, the Ohio supplies the largest 

 amount, and is subject to the greatest variation ; but when the 

 flood comes, a thousand cubic feet per second extracted from one 

 river diminishes the flood exactly the same as if taken from another. 



If the Missouri River be relieved of the enormous quantities of 

 mud supplied to it by the bad-land and sand-plain rivers, it will 

 cease to cut its own banks, and will discharge its waters into the 

 Mississippi, destitute of the sediment coming from these tributaries, 

 and also destitute of the sediment derived from lateral cutting. 

 When the waters of the Missouri are thus delivered to the Missis- 

 sippi in a comparatively pure condition, they will cease to choke the 

 Mississippi ; and the clearer waters of the combined Missouri, up- 

 per Mississippi, and Ohio, flowing in one volume as the lower Mis- 

 sissippi, will be able to keep its channel unobstructed. 



It will now be readily understood why the storage of the head 

 waters of the Missouri and other western tributaries, and their di- 

 version for the purposes of irrigation, will result beneficially to the 

 agricultural interests and to the navigation of the lower Mississippi. 

 The advantages to navigation and the immunity from floods made 

 possible by storage reservoirs alone are well known ; but there 

 should be added to these benefits that which comes from depriving 

 the stream of its chief instrument of corrasion, namely, sediment. 

 Such reservoirs should be constructed along the tributaries of the 

 Missouri, which, as we have seen, is the main source of the sedi- 

 ment-supply of the Mississippi system. Fed by the drainage of the 

 steep slopes of the Rocky Mountains and the bad-lands and the 

 sand-fields of the Great Plains, the waters of the Missouri come 

 loaded with the materials which go on cutting and grinding with 

 constantly increasing energy in their journey to the sea, choking the 

 channel and cutting away the land. Imprison these waters in set- 

 tling basins, divert them to the purposes of irrigation, and they are 

 robbed of their destructive agency. 



It is not maintained that such storage and irrigation works 

 will entirely supplant other resources of the engineering art (revet- 

 ments, wing-dams, jetties, etc., will still have their uses), but the 

 principal difficulties in the way of the successful application of 

 these resources will disappear with the establishment of the work 

 proposed ; and, until such works are constructed, the secondary 

 agencies for the control of the river will be useless. 



The waters which are precipitated on the Rocky Mountains, and 

 which roll over the sands and bad-lands of the Plains, are those 

 which directly and indirectly load the Mississippi with its supera- 

 bundant sediment. These waters are all needed in the arid lands 

 through which they flow, that such lands may be redeemed by ir- 

 rigation to agricultural purposes. The sediment which they carry 

 can be poured on desert wastes, and render them fertile ; and the 



