142 



INDIANA UNIVERSITY STUDIES 



'teresting fashion. McBrides Creek flows through this vaUey and 

 furnished part of the sediment. 



Bank Cutting in General. Any obstacle or obstruction on or 

 near the bank of a stream causes the current to be deflected to the 

 opposite side of the channel, where it begins to cut away the bank, 

 and is again deflected back to the side where it first started. This 

 is the beginning of a meander. 



Bank cutting causes the velocity of the current to be retarded 

 on account of the increased friction. The increased length of 

 the course as well as the increased load also reduces the velocity 

 of a stream. These conditions, resulting from bank cutting, all 

 tend to reduce the velocity of the stream, thus making the flood 

 stages higher. As a rule, a flood plain is made up of materials 

 that are easily eroded or moved. A great many observations 

 along the river showed that the top soil was from one to ten feet 

 in thickness, while the under layers were composed of sand and 

 gravel. This sand and gravel is easily moved by the current which 

 washes it out from beneath the top soil or loam, permitting the latter 

 to cave in. This accounts for the very steep banks on the out- 

 side of the meanders, and for the rapidity with which the current 

 removes the material from the outside of the meander. The sand 

 and gravel is carried to the inside of the meander and deposited 

 in the form of sand bars; this is done in a large measure by the 

 cross currents. The soil is lighter and is carried farther down 

 stream and in many cases carried out and deposited on the flood 

 plain. The sand and gravel has been moved from one side to 

 the other many times. Many beautiful cross sections of large 

 sand bars were seen. Figure 15, shows a typical vertical out- 

 side bank of a meander. It seems that the shifting of the stream 

 goes on more rapidly when the bank is just full or only partly full 

 of water, for when the water is over the banks that which is left 

 in the old channel seems to have less erosive power, or at least 

 not any more than when the bank is just full. Even if the rate of 

 bank cutting were the same during the over-flow stages of the 

 river as when the banks are just full, the latter conditions occur 

 much more often than the former, and it is therefore evident that 

 there should be some measures taken to prevent rapid formation 

 of meanders. 



Eflfeet of Trees on Bank Cutting. Trees along the river bank 

 will to a great extent retard bank cutting. Sycamores and willows 

 are possibly the best for this purpose. Figure 13 shows the roots 



