THE WORK OF RUNNING WATER. 



195 



now to be added that they periodically deepen their channels to a not- 

 able extent, and that the deepening of the channel takes place at the 

 very time when the flood-plain is being aggraded. In other words, 

 the stream in flood aggrades its plain, and degrades its channel. This 

 follows from the fact that the current is sluggish in the former position, 

 where the water is shallow, and rapid in the latter, where it is deep. 

 When the flood subsides, the channel, deepened while the current was 

 torrential, is filled again by the feebler current which follows. This 

 alternate deepening and filling is known as scour-and-fill. It is well 

 illustrated by the Missouri River. At Nebraska City, scour is believed 

 to occasionally reach depths of 70 to 90 feet.^ At Blair, about 25 miles 

 above Omaha, the same river is believed to cut to bed-rock (about 

 40 feet below the bottom of the channel in low water) twice a year, 

 that is, during floods.^ Fig. 185 shows the changes recorded in the 





Fig. 185. — Diagram illustrating scour and fill in the Missouri River. A record of 

 soundings at Blair Bridge (near Omaha), 1883. Shows also the cross-sections of 

 the river at various rates. (Todd, Bull. 158, U. S. Geol. Surv.) 



channel of the river at this point during the year 1883. It shows that 

 the scour-and-fill during this year amounted to almost 40 feet. All 



1 Cooley. Rept. U. S. Engineers for 1879-80, Pt. II, pp. 1060 and 1071. 



2 Gerber. Cited by Todd. Bull. 158, U. S. Geol. Surv., pp. 150, 151. 



