The Behavior of Rivers 35 



struction, acting as a hammer in striking and breaking and 

 grinding the hardest rocks. 



The California that we know is above the sea. Some parts 

 of the State have been above sea level and subject to the proc- 

 esses of erosion just mentioned for long periods. Portions of 

 the State have been very recently lifted above the level of the 

 waves. Part of what we call California is below sea level, as 

 witness the islands off the coast which are mountain tops that 

 project above the surface of the sea from a continental shelf. 

 Rivers, some of them mighty rolling hammers, traverse the land 

 surface. Large or small, they are all working at destruction 

 of the land, and carrying broken rock toward the sea. Some 

 are mighty rolling reservoirs whose currents are sluggish so that 

 they deposit their burden of sand and mud and build up plains 

 which may in time of flood be again torn up and carried farther 

 on toward the sea. Some "rivers" are rivers in name only ex- 

 cept in time of deluge from rain or melting snows. "Dry 

 creeks" become raging torrents that move with terrific power 

 and great destructiveness. The streams are all agents of de- 

 struction. Large or small, crossing broad plateaus with con- 

 stant flow or sinking into the parched soil of the arid desert, or 

 hammering down mountain slopes through deeply eroded can- 

 yons, all are working at the great task of wearing down the 

 land, tending to reduce mountain, plateau and hill to a plain, 

 and ultimately transferring the rocks of the land to the sea 

 bottom, there to again become rock formations. 



How Rivers Work 



It is well to inquire how rivers behave. The methods by 

 which running water does its work may serve to explain much 

 of the geology of this great and vastly varied State. The geol- 

 ogy of California is very complicated. There have been many 

 upheavals; there have been many down- throws or depressions 

 (subsidences) . But during all the long geologic ages running 

 water has been continually and persistently at work. Every 



