10 BULLETIN 54, U. S. DEPARTMENT OF AGRICULTXTBE. 



belong the present and former drainage systems of the Amargosa 

 and Mojave Rivers. 



The Great Basin contains but one other major division. North- 

 west of the Lahontan Basin, where the zone of upUft and fracture 

 extended into the great lava plateau of eastern Oregon and north- 

 eastern California, a number of small basins were produced. Some 

 of these drain or have drained to Lahontan, some to the sea, and 

 some have been permanently inclosed. 



Outside the Great Basin there is but one inclosed area where 

 structure appears to have controlled the restriction of the drainage. 

 This is the great trough between the Sacramento and San Andreas 

 uplifts in central New Mexico, and extending southeastward into 

 western Texas. Even here the structural character of the basin is 

 far from certain, as will appear when the region is discussed. The 

 only other large and well-loiown basin is the Salton, in southern 

 California. It, too, occupies a structural trough which is, however, 

 open to the sea, the only barrier being an alluvial dam apparently 

 built by the Colorado River. 



Though the above statements cover all important structural basins 

 and all which have attracted any considerable attention, there remain 

 numerous and extensive areas where seaward drainage has ceased 

 because of the decay and contraction of the river systems. These 

 areas are of considerable diversity, but fall well into geographical 

 groups and will be so discussed. 



In the detailed discussion which follows all undrained basins 

 of the United States wUl be treated under the following groups : 



(1) The Lahontan Basin and its tributaries. 



(2) The Bonneville Basin and its tributaries. 



(3) The basins of the Lava Plateau. 



(4) The trough valleys of Nevada and the basins of the 



Transition Zone. 



(5) The trough valleys of California and the Mojave Desert. 

 (6)- The Salton Basin. 



(7) The basins of the New Mexico-Texas trough. 



(8) The trough valleys of Arizona and Sonora. 



(9) The Lordsburg-Membres region (New Mexico) and the 



Chihuahua bolsons. 



(10) The Rocky Mountain basins. 



(11) The Great Valley of California. 



(12) The filled lakes of the California ranges. 



(13) The basins and ponds of the Colorado Plateau. 



(14) The ponds and coulees of Eastern Washington. 



(15) The ponds of the Great Plains. 



(16) Local basins of unusual origin. 



