APPLICATION TO NATURAL STREAMS. 



221 



is affected by redaction of velocity. There is 

 also diversity in the directions followed by 

 elements of current, and this diversity includes 

 not only the twisting movement but various 

 minor eddies and swirls. Diversified move- 

 ments, by including upward movements, pro- 

 mote suspension, and in conjunction witb 

 diversified velocities they modify the partition 

 of the load between traction and suspension. 

 On the whole, suspension claims more in a 

 diversified current, but it is also true that the 

 line of separation between suspension and 

 traction shifts to and fro in such a current. 

 Much debris which is suspended in the swift 

 water under the concave bank joins the bed 

 load in passing the shoal between deeps, and 

 the suspended load is still more restricted in 

 passing the shoal of the convex bank. Deposi- 

 tion on the latter shoal includes both tractional 

 and suspensional materials. 



FEATURES CONNECTED WITH DIVERSITY OF 

 DISCHARGE. 



All streams vary in volume from season to 

 season and from year to year. In a stream fed 

 by springs the changes may be slight. At the 

 opposite extreme are creeks and even rivers 

 which exist only during storms. In most large 

 streams the discharge at flood stage is many 

 times greater than at low stage. Usually flood 

 stages continue only for brief periods and in the 

 aggregate occupy but a small fraction of the 

 year. 



It is broadly true that streams give shape to 

 their own channels, and among alluvial streams 

 there are few exceptions. It is broadly true 

 also that the shapes of channels, including cross 

 sections and plans, are the same for large 

 streams as for small. But the large stream 

 requires and develops a larger channel broader, 

 deeper, and winding in larger curves. Through 

 variation of -discharge the same stream is alter- 

 nately large and small, so that its needs are 

 different at different stages. At each stage it 

 tends to fit its channel to the needs of the par- 

 ticular discharge. The formative forces resid- 

 ing in the current are so much stronger with 

 large discharge than with small that the greater 

 features of channel are adjusted to large dis- 

 charge, and this despite the fact that floods 

 are of brief duration. The feebler forces of 

 smaller discharges modify the flood-made forms 

 but do not succeed in completing their work of 



adjustment before it is interrupted by another 

 flood. The deeps of high stage are pools at 

 low stage and have currents too feeble for trac- 

 tion. As the reduced stream passes from pool 

 to pool it crosses the shoal formed at high 

 stage with quickened current. The velocities 

 are still diversified, but the greater and smaller 

 velocities have exchanged places. The slope of 

 water surface is more diversified than at high 

 stage, being lower at the pools and higher be- 

 tween them. Traction is restricted to the 

 shoals, and the loads are small. The load at 

 each shoal is obtained from the shoal itself and 

 is deposited in the next pool, and in this way 

 shallow channels are developed from pool to 

 pool. 



In contrasting the features of high and low 

 stages, it has been convenient to use the terms 

 as if high stage and low stage were specific and 

 definite phases of stream activity, thereby 

 ignoring the actual diversity in fluctuations of 

 discharge. Floods are of all magnitudes, and 

 each flood presents not only a maximum dis- 

 charge but a continuous series of changing dis- 

 charges. At each instant the stream contains 

 a system of currents of which the details 

 depend not only on the discharge but on the 

 shapes of channel created by the work of pre- 

 vious discharges. So long as the discharge 

 continues, its currents are eroding and deposit- 

 ing in such way as to remodel the channel for 

 its own needs, and so long as the work of 

 remodeling continues the loads and capacities 

 at different cross sections are different. 



With the changes in the values and distri- 

 bution of velocities go changes in those values 

 of competent fineness which on one side limit 

 traction and on the other separate traction 

 from suspension. With maximum discharge 

 all the coarser grades of d6bris within the 

 domain of the stream are in transit along the 

 path of highest activity, and that path in- 

 cludes the deeps and the intervening shoals. 

 With lessening discharge the coarsest material 

 stops, but it stops chiefly in the deeps, because 

 the change in bed velocity is there greatest. 

 At the same time the coarsest of the suspended 

 load escapes from the body of the stream and 

 joins the bed load. By this double change the 

 mean fineness of the tractional load is increased, 

 and so also is the mean fineness of the suspended 

 load. With continued reduction of discharge 

 the tractional load in the deeps becomes gradu- 



