Ch. 18] STREAM-CHANNEL PROCESSES 321 



nating from side to side, with the main current following a sinuous pat- 

 tern between them. Such sinuosity of flow within a relatively straight 

 channel, called serpentining (Schoklitsch, 1937, pp. 148-149), may be 

 observed in many straightened stream channels at low water. 



Once lateral bank erosion starts, for any reason, and the channel is 

 locally widened, the process is self-sustaining. The recession of the 

 bank permits the current to wash more directly against the down- 

 stream side of the eroded area, and thus the erosion and bank reces- 

 sion are continued. Sediment accumulates on the opposite side of the 

 channel, or inside of the bend — the result of lessening velocity at that 

 place as the main current turns away — and the channel gradually 

 shifts in the direction of bank attack. This is the process of meander- 

 ing, which is responsible for most of the natural instability which 

 channel-control works are intended to check or correct. 



The term "meander" is usually applied to the relatively smooth, 

 regular loops developed b}?- a stream where the rate of deposition on the 

 inside of the curve is approximately equal to the rate of erosion on the 

 outer side, so that the channel does not change greatly in width but is 

 shifted into a longer, curving course. This has been called "forced-cut 

 meandering" (Melton, 1936) . The process is basically the same when 

 deposition fails to keep pace with the lateral erosion, or "advanced-cut 

 meandering," but the effects are quite different, for the channel is wid- 

 ened and the opposite banks may become quite diverse in plan. Prob- 

 ably all natural streams tend to meander, but the type and extent of 

 meander development vary widely. Figures 1 and 2 illustrate short 

 sections of the Missouri River as it was in 1890, when advanced-cut 

 meandering was prevalent, and in 1945, after artificial bank revetment 

 and training dikes had restricted the rates of bank erosion and regular- 

 ized the channel into a "forced-cut" meander pattern. 



Stream meanders usually occur in series, for the development of one 

 meander or bend tends to direct the current against the opposite bank 

 below and thus initiate a second bend or meander, and so on down the 

 stream. The continued growth of one meander bend also causes a 

 progressive downstream shift in the point of incidence of the current 

 against the opposite bank at the head of the next bend, and thus there 

 is normally a downstream progression or sweep of bends, or meander 

 loops. Figure 1 shows that Saline City Bend on the Missouri River 

 migrated more than a mile downstream from 1890 to 1945, and Rush- 

 ville Bend, shown in Fig. 2, migrated nearly two miles in the same 

 period. Further migration of Rushville Bend obviously would destroy 

 a section of railway, unless artificially restrained. 



