%7^ GEOLOGY. 



to remain overloaded. A stream may become overloaded in various 

 ways. It might at first seem unnecessary to inquire whether a stream 

 may be overloaded at its source, but the question is not necessarily to be 

 answered in the negative. The source of a stream is not always a 

 definite point. In a general way it may be said that the source of 

 the normal stream is at that point in its valley where the bottom is 

 as low as the ground-water level of the region. But since the ground- 

 water level is not constant (p. 71) the source of a stream is likely to 

 be farther up its valley in a wet season than in a dry one (p. 72). After 

 a heavy shower, the run-off descends to the axis of the valley from the 

 slopes on all sides, and temporarily the stream begins above the point 

 which marks even its wet-season source. If under such circumstances 

 the slopes about the head of the valley are notably steeper than the 

 slope of the valley itself, as they frequently are, the water flowing 

 down them may gather an amount of material which it cannot carry 

 after it reaches the bottom of the valley. This may be the case at, 

 or even above, the point which marks the source of the permanent 

 stream. It is, therefore, possible for a stream to be overloaded at 

 its source, if we take the source to be the point whence the water per- 

 manently flows. Deposition may, therefore, be taking place in a 

 valley at the head of its permanent stream, or temporarily even in the 

 valley above it. 



Streams issuing from glaciers sometimes have more load than they 

 can carry after they escape from the ice. If the stream be regarded 

 as beginning at the point where it issues from beneath the ice it may 

 be overloaded at its source.^ 



Under certain circumstances, a stream may overload itself. Thus 

 if a stream loaded with coarse detritus reaches a portion of its valley 

 where fine material is accessible in abundance, some of the velocity 

 which is helping to carry the coarse may be used in picking up and 

 carrying the fine. This reduces the velocity, and since the stream 

 already had all the coarse material it could carry, reduction of velocity 

 must result in deposition. It follows that when a stream fully loaded 



* A question might be raised in this case as to what should be called the source. 

 A spring issues from beneath the surface and flows away in a stream. The stream 

 is said to begin where the water appears at the surface, though in some cases the water 

 of the spring was a sub-surface stream before it reached the surface. Water escaping 

 from beneath a glacier as a stream may likewise be considered a spring at the point 

 of its issue. 



