THE WORK OF RUNNING WATER 



99 



If, later, the excess of sediment ceases, the stream sets to work to 

 remove that which was temporarily laid aside in its flood plain. 



River Lakes 



Rivers tend to drain the lakes of high lands, but they make 

 lakes both in their flood plains and at their debouchures. Oxbow 

 lakes have already been mentioned (p. 88), but river lakes arise 

 in other ways as well. A tributary stream, with a high gradient, 



Fig. 106. Lake Pepin, a widened part of the Mississippi River between Wis- 

 consin and Minnesota. Maximum width about 2^ miles. The widening 

 of the river is apparently due to the detritus brought down by the Chip- 

 pewa River, and deposited in the Mississippi. 



may bring more sediment to its main than the latter can carry away. 

 That which is deposited may form an obstruction in the channel of 

 the main stream, ponding the water above. A river broadened in 

 this way is often called a lake. Lake Pepin in the Mississippi River 

 (Fig. 106) is an example. 



Rafts of timber are sometimes formed in rivers, and these rafts 

 may obstruct drainage, ponding the waters both of the main stream 

 and of its tributaries. A huge raft of this sort developed in the Red 



