14 
BULLETIN 1486, U. S. DEPARTMENT OF AGRICULTURE 
(4) Where there is a soft bottom and the depth at flood stage is 
very large in proportion to the normal depth. 
In explanation of the last item, for streams (like the Colorado 
River) having a very soft bottom and wide fluctuation of stage the 
bottom is constantly moving. As the stream rises the increased 
bottom currents erode and pick up the bed material, dropping it as 
the flood crest recedes. For conditions of this kind the bed of the 
stream may move downward as much as 2 feet for every foot rise 
in water stage. This movement of the channel bottom is distin- 
guished from other erosive action in that the movement is a constant 
fluctuation — in fact it may be termed an " elastic movement" of the 
channel, to accommodate itself to changing hydraulic conditions. 
(5) Where there is a constriction of the waterway coupled with a 
bottom of erodable material the power of a stream to erode varies 
as the square of its velocity so that any channel constriction oper- 
ating to increase the current velocity increases the eroding power of 
4 Pf / BAR / 
.FLOOD STAGE CURRENTS CUT ACROSS BAR 
AND DEFLECT MAIN CURRENT AGAINST BANK 
AS SHOWN 
Fig. 14. — A typical example of normal and flood current action at a bend in a stream 
the stream. In general, every material has a definite velocity at 
which it starts to erode. Silt and fine sand, moving at a compara- 
tively low velocity, gravel at a higher velocity, and so on. 
For shallow streams, soft silt bottoms will generally erode at a 
bottom velocity of less than one-half foot per second, sand bottoms 
at a velocity of from 1 to 1.5 feet per second, ordinary clay at a 
velocity of about 2 to 3 feet per second, compact clay at a velocity 
of 5 to 6 feet per second, and heavy gravel at a velocity of about 
4 to 6 feet per second. 
(6) At points where there is bend in the channel (fig. 14) which 
directs a current of considerable velocity against one pier or abutment. 
SHIFTING OR LATERAL MOVEMENT OF CHANNEL 
A crossing site to insure stability of the stream channel should, 
in general, be at a point where the stream runs between well-defined 
banks. These banks must be of comparatively resistant material. 
