84 NATURE STUDY REVIEW [9 :3— Mar., 1913 



natural levee, and after that a broad flood-plain. The steep bank 

 was on the outside of the bend in the stream. 



A river carries its sediment according to the amount of sur- 

 face in comparison to the weight. The heavier load is rolled along 

 the bottom of the river, while the lighter is carried suspended in 

 the water by means of small currents made by boulders or 

 irregularities in the surface of the stream bed. 



The amount of load a river carries greatly controls its cut- 

 ting ability. Sediment is the tool of a river in erosion. The 

 river got most of its load from banks and the stream bed. 



The growth of a gully was very plainly illustrated on the 

 Thornton trip. One of the principal ways of a gully growing is 

 by head erosion. The stream soon cuts back and the property at 

 the gully's head will be broken up. We saw a good example of 

 this. A farmer wished to drain his land and so dug a ditch which 

 he connected with the head of a small gully which was on his 

 property. When we saw the place the gully had been cut back 

 so far that a great hole had been made in the farmer's corn field. 

 The addition of the extra water from the draining ditch had 

 hastened the development of the gully very much. 



Another way of gully growth is by widening. The stream 

 does this by undercutting its bank, and with the help of ground 

 water, which is very instrumental in making slumps and creeps, 

 the banks fall slowly into the stream and are carried away. As I 

 have said before, the current is stronger on the outside of a bend 

 than on the inside, and therefore, if the river meanders, the work 

 of side cutting will go much faster. An illustration of cutting on 

 the outside of a bend was shown on this trip by seeing a farm 

 house on the edge of a cliff-like bank. The farmer who owned 

 the house said, that a few years ago when he had come to live 

 there, a grove of trees separated the house from the river. Now 

 he expected to see the house, too, go soon. 



The third way a stream enlarges a gully is by erosion on the 

 bottom. When the stream carries much heavy sediment which is 

 rolled along the bottom the stream bed is quickly deepened. 



When a river has widened its channel enough to meander 

 it has a flood plain which is usually very fertile. When a stream 

 is near enough base level to have large meanders, in times of 

 flood the swift current often cuts off one of the meanders by 

 making a new channel through the narrow divide. The elim- 

 inated meanders are filled with water and because of their form 

 are called ox-bow lakes. 



Terraces are formed to a large extent by a meandering 



