BYBEE-MALOTT : THE FLOOD OF 1913 



175 



Bank-Cutting 



Several times in this report, bank cutting has been partic- 

 ularly mentioned as an important phase of the flood situation. 

 Mention has also been made of the fact that bank-cutting is not 

 confined to flood stages, but to stages when the water is three 

 or more feet above low water mark. As a rule, the bottom of the 

 river channel and the lower part of the banks is somewhat tougher 

 than any part above. No bank cutting goes on in the low water 

 condition; but as soon as the water has risen three or four feet 

 in the channel, it begins to come against the outside bank of the 

 river in rounding a meander, and comes in contact with the looser 

 material above the tough, compact, lower part. Caving is then 

 an immediate result. As the water rises higher, it gains in velocity 

 and its efficiency for bank cutting increases until the channel is 

 bank full. This is probably the most favorable condition for bank- 

 cutting, for, as soon as the water begins to flow over the valley 

 land, across the neck of the meander, some of the force of the cur- 

 rent is taken in the direction of the overflow, and the velocity of 

 the current is checked, thereby lessening the cutting power. More- 

 over, when the water rises high over the valley land, the thread 

 of swiftest flow is raised, perhaps, above the banks and bank-cut- 

 ting is lessened. 



It is interesting to note the relation of the height of the river 

 banks to the width of the valley. On the West Fork above Gosport 

 and on the East Fork above Sparksville, the valleys are from one 

 to four miles wide, due to the very susceptible erosiveness of the 

 Knobstone Group of Rocks. In these regions the banks are low, 

 ranging from six to twelve feet above low water mark. From 

 Gosport to Worthington, on the West Fork, and from Sparksville 

 to the southwestern corner of Martin County, on the East Fork, 

 the valleys range from less than a quarter of a mile to a mile in 

 width, due to the highly resistant erosiveness of the Upper Mis- 

 sissippian rocks and the Mansfield sandstone of the Lower Penn- 

 sylvanian rocks. The banks in these regions are from twelve to 

 forty feet above low water mark. The remaining parts of both 

 forks are in the easily eroded coal measures, and the valleys are, 

 therefore, wide. Again, the banks are low, ranging from eight 

 to fifteen feet in height. Thus, in the wide valley regions, the 

 river banks are low, and in the restricted valley regions the river 

 banks are high. 



The above conditions and relations are easily explained. In 



