November i6, 1888.] 



SCIENCE. 



231 



beds are horizontal the stream slowly passes from one bed to 

 another by reason of its declivity, and when the dip of a formation 

 coincides with the declivity of a stream, the change which arises in 

 passing from one formation to another is reduced to a minimum. 



Heterogeneity of terrain has an important effect upon corrasion. 

 Hard beds are corraded with difficulty, soft beds with ease. By 

 this means the channel is broken into sections or reaches, now 

 shorter, now longer, with the varying heterogeneity of the terrain, 

 so that soft beds present reaches of lower declivity and hard beds 

 reaches of higher declivity. The low-declivity reaches are expanded 

 and the high-declivity reaches are contracted. Where the changes 

 are more abrupt the declivity becomes more abrupt, so that the 

 stream may be made to plunge in a part of its course and to flow 

 gently in another part. The efficiency of corrasion is greater in the 

 softer reaches, but the corrading power of the stream is increased 

 with declivity, and thus the corrading power is concentrated on the 

 harder reaches. 



Under the conditions so briefly set forth, the smallest stream has 

 a more or less heterogeneous terrain, and a great river like the Mis- 

 sissippi possesses a terrain of indescribable heterogeneity. 



As maximum corrasion is along the line of miximum flow, pro- 

 gressive deepening of channel produces progressive narrowing of 

 the channel ; but this tendency is counteracted in various ways. 

 The narrowing of the channel is checked by the instability of the 

 banks. If the banks are greatly coherent, long continued corrasion 

 may result in the formation of deep canons. As cohesion becomes 

 less, the banks fall into the stream and the channel is widened ; and 

 when the terrain is composed of disintegrated materials the chan- 

 nel is widened until the banks have the normal slope. If the terrain 

 is permeable to water, the material creeps into the channel and the 

 slope is still further reduced. Quicksands that become saturated 

 to the level of the stream, flow out, and excessive widening results 

 therefrom. In the progressive lateral corrasion of an alluvial bank 

 by the impact of the stream which is turned against it, wetting 

 secures disintegration, and the banks progressively fall into the 

 water. When the wetted perimeter — which is that portion of 

 the channel surface covered by water — is below the channel per- 

 imeter, sapping results, and the load is still further increased. Given 

 sufficient time, indurated banks assume the normal slope. Alluvial 

 banks speedily assume this condition through the agencies which, 

 considered together, may be called ' weathering ; ' and through the 

 ever-recurring wash of rains the slope is ever diminished. In like 

 manner, the most indurated banks — as of basalt, granite, or lime- 

 stone — are reduced to low slopes. The stream corrading verti- 

 cally through indurated rocks steadily increases its vertical banks 

 below, while the weathering steadily decreases them above ; so that 

 the height of the precipitous portion of the banks is tlie residual of 

 opposing agencies. 



Corrasion is greatly modified by the declivity of the stream. This 

 declivity may be so great that no portion of the load is deposited 

 along its course. All the load is transported by flotation or driving. 

 Under these circumstances, if the terrain of the channel is suffi- 

 ciently coherent, the corrasion will be wholly vertical, and its rate 

 will increase with the declivity, as the impact of the corrading parti- 

 cles will be increased thereby. This vertical corrasion will produce 

 cafions with precipitous walls until at last cliffs thus formed will be 

 broken down by sapping. But when along the course of a stream 

 the declivity is diminished so that any portion of the load is de- 

 posited, such deposit serves to protect the bottom of the channel 

 and to check vertical corrasion, but at the same time the channel is 

 choked by the material thrown down, and the waters passing down 

 the channel are turned to one side, and lateral corrasion is inaugu- 

 rated thereby. In the same manner lateral corrasion is produced 

 by the sappmg of the cliffs, as the fallen cliffs choke the channel of 

 corrasion, and the river is thus turned against its banks, which are 

 high walls. Lateral corrasion therefore arises from local deposition 

 and from no other cause. 



If the declivity of the stream is diminished to such an extent as 

 to prevent vertical corrasion, whatever corrasion exists must be lat- 

 eral corrasion ; and here again, the greater the load the greater the 

 deposition, and the greater the resulting corrasion. 



When a river flows over a plain with declivity so low that verti- 

 cal corrasion is wholly checked, it is in a condition where lateral 



corrasion is at a maximum under the existing circumstances. This 

 lateral corrasion is greater as the load is greater. Other things 

 being equal, declivity determines whether corrasion shall be vertical 

 or lateral, through the intervention of deposition. 



In vertical corrasion the load is the instrument by which the 

 channel is abraded, and in lateral corrasion the load is still the in- 

 strument with which the work is performed, but it is used in a two- 

 fold manner: (i) it is the agency by which the stream is turned 

 against its banks, and (2) it is still an instrument of abrasion. 



The volume of water is increased by every affluent : it is there- 

 fore progressively enlarged from source to mouth, and the con- 

 ditions of corrasion and transportation are greatly modified thereby. 

 At the junction of an affluent the volume of the stream is enlarged, 

 and the rate of corrasion is increased, vertical or lateral, or both. 

 If the declivity of the affluent is much greater than that of the prin- 

 cipal stream, the affluent brings with it load too coarse to be further 

 transported. In this manner the main river is choked, and is inter- 

 rupted by a series of dams constructed by the affluents. This is 

 especially remarkable in streams running in canons. Where these 

 conditions prevail, that form of the channel which is usually pro- 

 duced by heterogeneity of terrain — that is, by harder or softer 

 formations — is sometimes obscured, or even obliterated, by affluent 

 dams. The corrasion which results from increase of volume of 

 water and sediment causes the channel below the affluent to be cut 

 faster than the channel above. In this manner, ceteris paribus, a 

 stream decreases in declivity from source to mouth, and a ' normal 

 curvature of stream declivity ' is produced thereby. 



The volume of the stream is variable from time to time, as it de- 

 pends upon the fall of rain and the melting of snow. This variable 

 is great, as the flood-water volume may be many times that of the 

 low-water volume. Increase of volume which arises in this manner 

 manifests itself in part as an increase in cross-section and in part as 

 an increase in velocity, and the rate both of transportation and cor- 

 rasion is increased thereby. Corrasion and transportation are in- 

 creased by another condition that arises simultaneously with the 

 increase of volume. The rainfall which produces this increase also 

 produces surface erosion. Surface erosion is intermittent, as it is 

 caused only by the wash of rain and snow. When the storms come, 

 the load is increased at a much higher rate than the volume of 

 water in the stream. At low-water time the load is precipitated, 

 and clear water flows in the stream, so that corrasion, transporta- 

 tion, and deposition are reduced, or even suspended. At flood time 

 corrasion, transportation, and deposition are at a maximum. When- 

 ever an affluent receives a local rain the volume of water is increased 

 and the volume of load augmented at a still greater rate. When 

 such an affluent discharges into a main stream which is slightly or 

 not at all affected by the rainfall, the new load is at once thrown 

 down, and the affluent dam is increased. Affluent dams are 

 primarily formed by sudden decrease of declivity, and are greatly 

 enlarged by local increase of volume. The effect of affluent dams 

 is to stimulate lateral corrasion below. In a region of great decliv- 

 ity this is expressed in the widening of the channel ; in a region of 

 somewhat less declivity it is expressed in the enlargement of the 

 flood-plain ; and in a region of minimum declivity it is expressed in 

 changing the position of the channel. 



It has been shown that the channel of a stream is widened and 

 narrowed by varied conditions of terrain. In soft formations it is 

 expanded, in harder formations it is constricted. Another variable 

 arises through the agency of affluent dams, as already explained. 

 There is still another agency by which this heterogeneity is in- 

 creased. The grand terrain of a river is subject to deformations, 

 in such a manner that there may be upheaval in one part and sub- 

 sidence in another. Subsidence alone may produce an expansion of 

 channel at its locus, and upheaval may hold the waters back and 

 produce expansion of the stream above the locus of displacement. 

 By either or both of these methods the channels of great streams 

 are largely modified, and even lakes are produced thereby. And 

 streams are ponded by still other agencies that need not here be 

 described. Stream-reaches expanded in this manner become areas 

 of deposition where waters are largely discharged of load. In such 

 cases streams are deprived of the instruments of corrasion, and 

 corrasion is checked to the extent to which this is true. It has been 

 seen that steep local declivities are formed by indurated geological 



