This text is the fourth volume (in two parts) of a six-volume set of books that provides a comprehensive and integrated treatment of all the major rivers and estuaries of the contiguous United States. Both the physical and biological characteristics of pristine river environments are presented in detail. Part A is concerned with west of the Mississippi basin drainage and west of the Mississippi River. Part B is concerned with east of the Mississippi basin drainage, east of the river and the draining into the Gulf of Mexico.