334 FIRST YEAR SCIENCE 



have not as yet been able to reach. Almost the entire 

 surface is so intricately carved into drainage lines, that 

 wherever water falls it immediately finds a downward 

 sloping surface. The main stream by this time has prob- 

 ably smoothed out most of its falls and rapids and has de- 

 veloped long, smooth stretches. 



Here it is no longer cutting down its trough, but has 

 only sufficient slope to enable it to bear along its load of 



A STREAM WORKING BACK INTO AN UNDISSECTED AREA. 



waste. It here deposits upon its valley floor about as 

 much as it takes away. In this part of its course a river 

 is said to be graded. The longer a river flows undisturbed 

 by any deformation of its valley, the fewer falls and rapids 

 it will leave and the longer will be its graded stretches. 

 The Missouri River near Marshall, Missouri, is an excellent 

 example of a graded river. 



Sometimes a stream becomes so overloaded with detritus, 

 which it has acquired in a steeper part of its extent, or 



