94 WJMCGEE — SHEETPLOOD EROSION. 



trict the ranges are often interrupted and the "valleys" open one into 

 another directly toward the sea, while in all portions the larger water-lines 

 frequently trench the sierras and unite two or more " valleys " in a single 

 drainage basin. Typically the " valley " is simply a vast elongated plain 

 tilted sharply upward into bounding sierras on both sides and around 

 one end ; the plain is slightly concave, and if the slope exceeds say 15 

 or 20 feet per mile, its center is marked by a " wash " — a torrent perhaps 

 a mile wide after the great storm, a sterile sand waste at other times; 

 and the upturned rim is carved by labyrinths of barrancas, many of 

 which unite at the mountain base to form shallow arroyas meandering 

 the plain, but seldom reaching its center. Thus the typical " valley " is 

 waterless save during storms, and its ephemeral waterways are multi- 

 tudinous in the bounding mountains and few or none in its flattened 

 interior. In the eastern part of the district, where the parallel ranges 

 are higher and lie closer together, some " valleys " carry permanent 

 streams northward and southward from the subcontinental divide toward, 

 and at storm time into, geographically great but hydrographically puny 

 rivers — the Gila in Arizona, and the Altar (or Asuncion, or Pitiquito, or 

 Magdalena, or San Ygnacio) and the Sonora, as well as the northern 

 branches of the Yaki, in Sonora. These " valleys " occupied by perma- 

 nent rivers are not typical for the district, in that they are rather broadly 

 flattened V-shape than concave in profile; yet they vary from normal 

 valleys in that water is scanty or absent in the depths and increases with 

 the ascent of the waterways and culminates well toward the summits. 

 Such "valleys," or the greater waterwa}^ by which they are connected, 

 ultimately trend westward through the bounding ranges and down the 

 general slope toward the sea — a bourne which they (excepting a tribu- 

 tary or two of the Gila and two more of the Yaki well toward the high 

 Sierra) never reach. So the important constituent elements of the dis- 

 trict are valley-plains, bounded by barranca-scored mountains and some- 

 times faintly inscribed with drainage lines which are nominally rivers 

 but actually sand wastes during 360 days of the average year; and the 

 mountain is far subordinate in importance to the valley-plain, while the 

 " river," though a necessary descriptive term, is little more than an 

 empty name. ' ' 



The general topographic relation of " valley," mountain, and " river '•' 

 is modified and often masked by a subordinate relation growing out of a 

 northeastward migration of the divides. It is to this migration that the 

 westward deflection of the principal rivers is due, and it is to the same 

 cause that the union of neighboring " valleys " into irregular basins must, 

 in most instances, be ascribed. So common is this tendency that, ex- 

 cept in the foot ranges of the high Sierra, few of the drainage basins 



