10 BULLETIN 54, U. S. DEPARTMENT OF AGRICULTURE. 
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 uplift 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-known 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 basis 
of the United States will be treated under the following groups: 
(1) The Lahonian Basin and iis 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 Menxico-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 coulées of Eastern Washington. 
(15) The ponds of the Great Plains. 
(16) Local basins of unusual origin. 
