SOUTHERN CA1,IF0E^^A. 81 



proper season, large patches of annual grama grass relieves the desert of its barren aspect, and 

 transfers the mind to scenes of neatly trimmed pleasure grounds set off with verdant shrubbery. 

 The next stretch mounts again to the pebbly strewn table-land of the desert, from which 

 you descend further by the steep sandy bluff which bounds the bottom land of the Colorado 

 river. 



STKEAMS AND WATER-COURSES. 



A consideration of the character of the various streams and water-courses in this region 

 belongs properly to the view of its external features, and derives especial interest from the 

 intimate relations they sustain to climate and agricultural resources. On the western slope the 

 various streams, each draining a very limited area, are remarkable more for their number than 

 their magnitude. Having their main sources near the mountain summit, they pursue their 

 tortuous course towards the sea, following all the irregularities interspersed by the separate 

 mjountain ranges and their projecting spurs. By these devious courses the descent is finally 

 accomplished without occasioning falls or cascades, which are so commonly associated with 

 mountain streams elsewhere. 



Their volume being necessarily dependent on the supply from local rains, they generally 

 attain their greatest bulk towards the close of the rainy season, when the melting snows at their 

 sources combine with frequent showers below to swell their volume. As the dry season ad- 

 vances they gradually contract their dimensions, till in the month of July most of the streams 

 near their mouth become absorbed in their porous sandy beds. The exception to this general 

 fact is seen only in those streams which, having their sources in the higher mountain ridges, 

 receive a sufficiently constant supply to exceed the amount lost by evaporation. 



The drying up of the stream beds is a gradual process, necessarily modified by the compara- 

 tive dryness of the atmosphere, as also by the relative absorbent or retentive character of their 

 beds. 



The point at which water ceases to flow is quite variable ; its more usual upward limit being 

 marked at or near the passage of the stream from the first rocky ranges into the Tertiary 

 formation. The point, however, as before stated, is by no means a fixed one ; thus, during the 

 night it extends further downwards thnn in the daytime ; in cloudy weather, for the same 

 reason, its course is more prolonged than under a clear sky. In the stream beds themselves, 

 however dry, water is generally found a short distance below the surface. 



The descent of these streams in the rainy season may be either a gradual process in the pro- 

 gressive saturation of their sandy beds, or the saturation being accomplished by previous 

 showers, the irruption may be sudden. A fine example of this sudden appearance was observed 

 in the San Diego river, in December, 1849 ; when, after a rainy night, by which its sandy bed 

 was completely saturated, the upper stream suddenly appeared in the form of a foaming body 

 of water, moving onward at the rate of a fast walk, curling round the river bends, absorbing 

 the pools, and soon filling its shallow bed with a brimming swift current. 



An instance of the more gradual descent was seen on the following season, December, 1850, 

 when, from the absence of local rain, its downward progress was slow and interrupted. 



The facts connected with this supply of running water seems to deserve particular attention 

 in this region, where its presence or absence is synouomous with barrenness or fertility. 

 11 M 



