HIGHWAY BEIDGE LOCATION 15 
Flood currents must be parallel to the bank lines unless the banks 
are high and of solid rock or other exceptionally resistant strata. 
The stream bottom must be fairly hard and the wet perimeter should 
be small in comparison with the cross-sectional stream area. The 
average depth should, in general, be large in comparison with the 
maximum depth. 
Stream development may be said to have its geologic origin in 
the formation of the first gulley or wash where water has fallen 
upon any hillside. As the run-off concentrates along lines of steepest 
slope, these gulleys are cut deeper and are extended first headward 
and later laterally. The extension of one gulley or gorge operates 
to intercept and cut off the drainage flow from adjacent, lesser 
gulleys, so that gradually out of the numerous small and independent 
drainage channels one principal system is evolved and thus a stream 
is formed. 
In the early or youthful stages of development the stream is 
characterized by its rapid fall, steep banks, comparatively few 
tributaries, and general tendency to erode or cut vertically. The 
geologist has designated streams in this stage of development as 
"youthful" streams and the drainage basin as "young topography." 
As the development progresses, the stream bed gradually cuts to 
grade, or base level, meeting the grade of its discharge outlet (sea 
level or lake level, as the case may be) at its mouth and extend- 
ing back therefrom in a form which approaches roughly a vertical 
parabolic curve. The slope along the upper reaches will therefore 
be comparatively steep while near its mouth the stream grade has 
flattened out, approaching a level. 
The power of a stream to erode varies as the square of the velocity 
for any given bed material, whereas the ability of a stream to trans- 
port the sediment which it has eroded varies as the sixth power of 
the velocity and also to a certain extent with the volume of stream 
discharge, a larger stream, being able to carry a greater sediment 
load than a smaller one, at the same velocity. The upper or steeper 
reaches of the stream, therefore, will be eroding and transporting 
sediment to such an extent as to bring to the less rapid lower river 
more load than it can transport so that some of the sediment may be 
deposited. Such a condition is indicated in Plate 2, B. 
As the stream continues to develop, lateral erosion tendencies 
widen the valley floor, forming flood plains of considerable expanse. 
The topography loses its original rugged outline, the stream becomes 
more sluggish, and the waterway and adjacent topography is said to 
have passed from youth into maturity. 
Streams which have eroded to base level and have, through a 
succession of years cut a broad flood plain, are very apt to be extremely 
unstable as regards lateral movement of channel. ' Such streams in 
general show a marked tendency to shift back and forth from one 
side of their flood plain to the other, as shown in Figure 15. The 
general tendency seems to be an oscillation back and forth from one 
side to the other in a series of broad bends which progress gradually 
downstream. The general sequence of stream movement in cases of 
this kind may be observed from a study of Figure 15. The original 
bank lines are shown by the solid lines in the illustration. During 
the first flood water the bank at D is quite apt to be eroded. As the 
stream rises, the velocity at D is increased, the cutting power is 
