SOURCES OP STREAMS. 97 



water is an exceptional, indeed an exceedingly rare, feature in the Sono- 

 ran district, and is hardly known save in its more elevated eastern por- 

 tion. * 



Sources of streams. — The sources of stream water are, as usual, three — 

 (1) melting snow on the high Sierra and the Mogollon, (2) ground-water 

 appearing as seepage or springs in the deeper valleys, and (3) the product 

 of rains. The first of these sources might be neglected, save that it con- 

 tributes (a) directly to the longer rivers, San Pedro and Santa Cruz in 

 Arizona, and Altar, Magdalena, Bacuache, Sonora, San Miguel, and the 

 main and minor branches of the Yaki in Sonora, and (b) indirectly 

 through ground-water to these and other streams during a part of the 

 year. 



The ground-water is of considerable importance as a source of streams, 

 partly since its flow is moderately steady, and its tendency is thus to 

 maintain the stream as such and to enable it to continue corrasion and 

 transportation, albeit feebly, throughout the year. In the mountain 

 gorges the ground-water commonly emerges as permanent or temporary 

 springs, while in the alluvium-lined valleys it simply seeps through the 

 sand, generally below the surface, though sometimes in a slender stream- 

 let winding through the broad sand wash. Among the rocks of the 

 mountains the ground-water movement is conditioned as in humid 

 lands, but in the broader valleys it is conditioned by a variety of fac- 

 tors, including the conformation of the alluvium-lined basin with its 

 various. arms and interruptions, as well as by the rate of evaporation, 

 etcetera. Thus the alluvial mass and the adjacent hard rocks are wetted 

 or saturated during the rainy season, and the water percolates down the 

 slopes along lines generally corresponding with those of surface drain- 

 age ; and wherever the surface drainageway is exceptionally deep, there 

 the ground-water most frequently emerges to flow for rods or miles, or 

 until evaporated or reabsorbed by the sands. So it happens that water 

 may often be found by digging in the dry sands of a wash ; that a nom- 

 inally permanent stream may appear, disappear, and reappear half a 

 dozen times in the course of a day's journey down a single storm-fash- 

 ioned wash, and that the rippling streamlets lengthen during the night 

 and in cool or cloudy weather and shrink during the day and the heat, 

 when evaporation culminates. 



It is by reason of this relation between land surface and ground-water 

 surface, in conjunction with the characteristic migration of divides, that 

 a considerable part of the Sonoran district is habitable by man, for it is 

 in the narrow gorges produced by the retrogressive erosion trenching the 



*The bounding rivers, Gila and Yaki, are maintained by tributaries from without the district. 



