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SCIENCE. 



[Vol. XII. No. 302 



formations and by affluent dams. The ponding of streams by dias- 

 trophic and other agencies tends to convert the rapids of such de- 

 clivities into cataracts. The plunging waters checked at the foot of 

 the declivity are loaded by the impact and thrown into gyratory 

 movements, as currents and whirlpools, and corrasion is thus greatly 

 intensified at that point ; and when the stream above is deprived of 

 its instrument of corrasion by ponding, the corrasion at the foot of 

 the rapids is much more intense that at the head, and thus it is con- 

 verted into a cataract. Where geological conditions for sapping 

 are most favorable, that is, where the strata are approximately hori- 

 zontal and composed of harder and softer material, the cataract 

 condition is still farther promoted. It is thus that heterogeneity of 

 width increases heterogeneity of declivity. 



When a stream deposits load, the place of deposit is governed by 

 a great variety of conditions. First, reaches of low declivity are 

 reaches of deposition, and in ponded reaches deposition is exces- 

 sive; Second, alluvial dams are sites of deposition; Third, when a 

 stream corrades vertically and cliff banks are sapped, the material is 

 at once deposited ; Fourth, when cliffs are formed by lateral corra- 

 sion in flood-terrains the load is carried down stream, but maximum 

 deposition occurs at the first quiet water below ; Fifth, deposition 

 sites are often adventitious. The most common agency of this 

 character exists in the flood-wood carried by the stream. A float- 

 ing tree may lodge below a reach of great lateral corrasion at the 

 head of a region of equilibrium, where there is no lateral corrasion, 

 and near to one bank. The tree so lodged may gather other drift- 

 wood, and inaugurate deposition, so that a bank will be rapidly 

 constructed and built up into an integral part of the terrain. By 

 this agency the stream may be turned against the opposite bank, 

 and a great river-curve with a radius of one or more miles may be 

 established, and thus square miles of the flood-plain may be cut 

 away through the accident of a lodging tree. 



It has already been seen how increase of load transforms vertical 

 corrasion into lateral corrasion. There are two noteworthy illus- 

 trations of this fact that may well be further explained. When the 

 stream debouches from a mountain course of high declivity to a plain 

 or valley course of low declivity, a part of its load is suddenly de- 

 posited, the channel is thereby choked, and vertical corrasion is 

 transformed into lateral corrasion. The stream is thrown against 

 one and then against the other bank, and a flaring gorge is pro- 

 duced with its opening to the valley side and its apex in the moun- 

 tains or hills. At the same time a broad, over-placed deposit is 

 made, called an alluvial cone or alluvial fan. In this case it will be 

 seen that as vertical corrasion is transformed into lateral corrasion 

 the total amount of corrasion is increased thereby. 



The trunks of great streams often run in low valleys where the 

 declivity is so slight that all corrasion is lateral. Here the load is 

 deposited, not uniformly throughout the channel, but wherever 

 there are reaches of quiet water. The deposition from the initial 

 load becomes less and less down stream from place of deposit to 

 place of deposit. At every place of deposit a bank or bar is 

 formed, which progressively becomes an impediment to the stream 

 and around which the stream is turned in a curve. As it is di- 

 verted from its course it strikes with force against the opposite 

 bank, and into this bank it corrades. If the banks are but slightly 

 coherent the stream is loaded again with new material. This is 

 greatly increased when the banks are cut in such a manner that 

 the method of degradation by sapping is initiated, and this sapping 

 is itself made more efficient if there are permeable strata into 

 which the water penetrates so as to assist in sapping. The new 

 load taken on in this manner serves further to choke the stream as 

 it adds to the deposits below. In this manner the stream is 

 turned against its banks at comparatively short intervals, and every 

 curve is increased. This agency for the increase of tortuosity is 

 counteracted by three other conditions that arise. First, two con- 

 tiguous curves in the same river may increase until they coalesce, 

 and a cut-off is established ; Second, the increasing tortuosity is 

 increasing length, and increasing length produces decreasing de- 

 clivity, and the local corrasion is diminished thereby ; Third, as the 

 river swings from bluff to bluff across its flood-plain, by lateral 

 corrasion, it works the materials over and over again, and grinds 

 and regrinds the materials composing the banks ; so that in the 

 general average, the alluvium of the lower reach of the flood-plain 



is more finely comminuted than the alluvium of the upper reach. 

 For this reason the load which is added from time to time in the 

 downward course of the stream is more and more commi- 

 nuted. Flotation is thus promoted ; that is, the particles are held 

 in suspension longer. The effect of this is that the particles make 

 longer excursions and therefore choke the river less, and gradually 

 they are robbed of their power of inducing corrasion. For this 

 reason, when all the conditions are present, rivers running through 

 low flood-plains become less and less tortuous as they approach 

 their outlets — a condition well illustrated by the Mississippi River 

 between Cairo and the Gulf. This law of tortuosity is interrupted 

 wherever a lateral stream brings coarse material into the reach 

 subject to the law. 



In a stream where corrasion is wholly vertical the deposited load 

 is driven along the bottom and reloaded from time to time, and the 

 channel is thus kept clear of fixed deposits ; but when a lesser de- 

 gree of declivity is reached, so that the deposits choke the channel 

 and cause lateral corrasion, the several deposits remain for a time 

 to be attacked by the stream, which shifts its channel gradually or 

 abruptly, as the case may be. When the declivity decreases to 

 such an extent that corrasion is wholly lateral, the deposits become 

 more permanent. A deposit once made is protected by subsequent 

 deposits, and the process continues until bars and banks are built 

 up into integral parts of the alluvial terrain. By this process the 

 stream is turned against some other portion of the terrain, and 

 loads itself again with new material. In the vicissitudes of chan- 

 nel-cutting recent bars and banks may sometimes be destroyed, 

 but very rarely. To a large degree the deposits become permanent 

 obstructions, continually increasing, until the river is wholly turned 

 out of its course. By this process the tortuosity is produced and 

 the channel is made to wander back and forth through the flood- 

 terrain. In one sense the whole flood-plain valley, or rather the 

 channel occupied by alluvial terrain, is the channel of the stream, 

 and is occupied in part by the river and in part by deposits. 



In this connection these two laws may be formulated : {a) When 

 sediment is deposited it ultimately causes other sediment to be 

 loaded ; (d) The wider the flood-plain in proportion to the volume 

 of the water, the greater will be the average length of time through 

 which each deposit remains in place. 



The forces of degradation are established by nature, and in gen- 

 eral cannot be increased or diminished by man, and he can only 

 control their operation to a limited degree. All of the forces in 

 hydraulic degradation are of vast magnitude, and are far in excess 

 of the powers actually utilized in the production of results, and 

 when man deals with them he deals only with conditions. To 

 make this clear to the mind, some illustrations may be useful. The 

 following may serve for this purpose. The flood-plain valley of 

 the Mississippi from Cairo to the jetties is about 550 miles in length 

 and about 49 miles in breadth ; that is, it has an area of about 

 27,000 square miles. If this flood-terrain be estimated to have an 

 average depth of 50 feet, it would give a geological formation of 

 about 250 cubic miles, which is wholly alluvial. 



The forces of erosion are chiefly found in the precipitation of rain 

 and snow from the heavens, and in the changes of the temperature 

 from hour to hour and from season to season. Now let us suppose 

 that all these forces could be utilized in the erosion of the described 

 Mississippi flood-terrain, as they are sometimes utilized in bad-land 

 hills, then the rate of erosion would be enormously accelerated. 

 To get a clearer conception of these conditions, suppose that the 

 whole flood-terrain were built into a system of hills having the nor- 

 mal slope of loose earth, and that between the hills there existed a 

 ramification of streams, as rivers, creeks, brooks, and rills, and that the 

 whole region was sufficiently elevated above the sea to give these 

 streams a rapid flow, and that the rainfall of the region remains the 

 same as at present, and that there be no protection from vegetation 

 or other agencies, — then the present rainfall would erode away 

 the described flood-terrain in less than five decades. Such condi- 

 tions are sometimes found in nature, though rarely. 



Now let there be built up in the mind a possible rate of corrasion 

 when the conditions for the highest rate are at a maximum. The 

 Mississippi River has been known to cut its banks at the rate of a 

 mile a month, and yet the river was not utilized to the extent of its 

 power ; in fact, but a modicum of its corrading energy was brought 



