THE WORK OF RUNNING WATER. 179^ 



with coarse material picks up fine, it becomes overloaded, so jar as the 

 coarse material is concerned. 



Again, tributaries may overload their mains. While tributaries 

 are usually smaller than their mains, they frequently have higher 

 gradients, and the smaller stream of higher gradient may bring to 

 the larger stream of lower gradient more material than the latter can. 

 carry away. Thus deposition may take place at the point of junction 

 of tributaries with their mains. This may go so far as to pond the 

 latter enough to cause its expansion into a river-lake. Lake Pepin, 

 in the Mississippi River at the mouth of the Chippewa (in Wis.), is an 

 example. 



Streams may become overloaded by losing velocity or volume, 

 or both. Decrease in velocity is brought about either by decrease 

 in declivity or in volume. In general, streams have lower gradients 

 and greater volumes in their lower courses than in their upper, arid 

 these two elements affect velocity in different ways. If the increase 

 in volume be not enough to counterbalance the decrease in decHvit^^, 

 as is often the case, a stream which is loaded in its upper course will 

 deposit in its lower. The decrease of velocity at the debouchure of a 

 stream almost always leads to deposition. 



Decrease in velocity as the result of decrease in volume is less 

 common. When decrease in volume occurs, it may be the result of 

 (1) evaporation, (2) the absorption of water into the bed of the streaiti, 

 or (3) branching — ^the giving off of distributaries. While evaporation 

 is going on everywhere, the diminution of a stream by this means 

 is usually more than balanced by the increase from tributaries, 

 rainfall, and springs; but in arid regions a very different condition of 

 things sometimes exists. If mountains in an arid region be capped 

 with snow, its melting supplies the streams during the melting season. 

 As the streams flow out from the mountains through dry regions, they 

 receive little or no increment from rainfall, tributaries, or springs, and 

 evaporation reduces the volume of water, or even dissipates it alto- 

 gether. Absorption of water into the bed of the stream often ac- 

 companies evaporation. Reduction of volume by evaporation and 

 by absorption is especially common in arid regions. Wherever loaded 

 streams are reduced in volume, whether by evaporation or absorption, 

 deposition takes place. 



