BIVER ACTION PHENOMENA 487 



in comparatively recent time. With this well established phenomenon there goes 

 several important corollaries : 



1. First, a depth of bed rock 100 feet below present low water does not demon- 

 strate a buried channel. The size of the present stream and its efficiency in time 

 of flood must be known before a conclusion can be reached. Moreover, as the 

 relative softness of rocks influences the rapidity of corrasion in the bottom, as well 

 as on the sides of the trough, greater depths are to be expected where the strata 

 are soft ; and, consequently, in the planation of a region by a stream there will 

 be considerable irregularity in the surface of the bed rock underneath the alluvial 

 deposits. 



2. In estimating the discharge of streams W'hicli have quantities of sand in their 

 trough, like the Missouri and Platte, the height of the water is only one factor, 

 and oftentimes not the most important factor for determining the cross-section of 

 the stream. Doubtless there will be less scouring out of the bottom in a straight 

 course than in a bend, but it would be very considerable, particularly where its 

 narrowness suggests a favorable opportunity for estimating discharge. The Blair 

 bridge is in a comparativeh^ straight part of the Missouri, yet its bottom cut out 

 30 or 40 feet in time of flood. 



3. The transfer of sediment down the valley of a river would be much more rapid 

 than commonly supposed. When the stream is at its full, the scores of feet in 

 thickness of material usually lying quiet in the bottom is moving with nearly the 

 full velocity of the surface of the flood. 



4. Recent objects may be buried to a depth of 100 feet within a few years. In 

 the shifting of the Missouri a dozen or score of years may suffice to transfer the 

 deepest point of a bend into dry land covered with ordinary vegetation. If a well 

 were sunk in such a place a brickbat or other recent ol>ject might be struck at that 

 depth. This shows how weak are some of the arguments for great antiquity of 

 objects found deeply buried in the alluvium of Louisiana, or the delta of the Nile. 



Mutual Flood- relief Channels 



When two streams unite which are subject to floods and one or both rich in 

 sediment, there is apt to be found a little above their junction a channel which 

 we will call a mutual flood-relief channel. It has long been recognized that when 

 either stream bears sediment, a bar is apt to be formed just above the junction of 

 the currents of the two streams. This may be ascribed maiuly to the diminished 

 velocity resulting from the collision and intermingling of currents. The swifter 

 stream is likely to be more heavily laden, and the resulting velocity, when it 

 mingles with the slower current, causes some of the sediment to drop ; hence the 

 point separating the streams tends to lengthen ; it tends to curve more or less 

 across the slower stream. This curve may be straightened when that stream 

 quickens its current in time of flood ; but, since the sediment tends to deposit 

 more rapidly near the current, there is apt to be left a depression separating the 

 elongated bar from the original bank. At times when either stream is in flood 

 and the other low, there is a tendency for the surplus w^ater to break over this sag 

 into the lower stream. This tends to keep it open at the level of high water at 

 least. Sometimes it is cut below low water, so that the bar forms an island. From 

 this explanation it will be seen that the erosion of this channel is especially 

 favored by floods occurring in the two streams at different times and not simul- 



