SWAMPS AND SHORES 475 



longer wet lands with open water, though they may afford rookery 

 sites to the aquatic birds which feed elsewhere. 



Shallow depressions in the glaciated surface in the north produce 

 more extensive and longer-lived bog conditions, ending in the familiar 

 peat moors of more northern regions. On the prairies, ponds and swamps 

 are familiarly known as sloughs, and disappear because of the en- 

 croachment of sedge and cattail rather than of bog moss. These cat- 

 tail marshes are rich in bird life, notably of the perching birds such 

 as the blackbirds and wrens mentioned above, besides a great variety 

 of waders and swimmers. Cattails supply food and nest material to 

 one of the most characteristic of the semi-aquatic mammals, the 

 muskrat. 



Bottom land marshes of rivers. — The history of river bottom 

 swamps and marshes is different from that of lakes, since wide flood 

 plains subject to inundation and retention of water are the end stages 

 of the erosion of all river valleys. Rivers are in general much older 

 than lakes; the river bottom and river delta swamps and marshes have 

 some continuity of geological history, and an older, more permanent 

 fauna. Furthermore, in the United States at least, lakes characterize 

 the north and rivers with wide lowlands the south. River bottom 

 marshes, in the southern states, with favorable climatic factors, have 

 a luxuriant vegetation, such as the canebrake of the lower Mississippi; 

 this tendency culminates in the tropics in the papyrus swamps of 

 Africa, the giant Arum of the Amazon, and the wild sugar cane of 

 New Guinea, 20 feet high. The lowlands of the lower Mississippi have 

 a characteristic growth of timber such as cypress, which grows habitu- 

 ally in the water, or the gums and other species which withstand long 

 immersion. 



The great river marshes are usually associated with the lower 

 courses of the streams; but this is not a necessary relation, for on the 

 Paraguay and the Nile vast open marshes characterize the upper or 

 middle parts of the river. Rivers flowing through a flat flood plain 

 frequently build up natural levees on their banks, which permit the 

 growth of timber. On the upper Paraguay this fringing strip of timber 

 is often so narrow that one sees through it to the open marsh and plain 

 beyond. A meandering stream cuts through these strips of timber so 

 that the former banks and parallel forest strips may be at right angles 

 to the existing course, the old stream bed being filled with herbaceous 

 marsh or floating plants. 



The bird life* is the most conspicuous element in the fauna in 



* Brehm 8 and Bengt Berg 7 describe the bird life of the White Nile, and Miller 8 

 and Roosevelt 9 give a view of that of the Upper Paraguay. The bird life of the 

 Mississippi bayous is described by Arthur. 10 



