LAYMEN KNOW OF SHEETFLOODS. 105 



stance the waters of a storm (apparently a local thunder-gust) had scooped 

 out a basin 25 or 30 feet wide, 10 or 12 feet deep, and 250 feet long, then 

 dammed its lower extremity with silty debris, and, finally, as the now 

 slackened, lined it with impervious silt which held water for months, and 

 so located a Papago settlement ; and cases were reported in which basins 

 or tinajas of this character, refilled from season to season for some years, 

 drew about themselves agricultural rancherias of Papago Indians, who 

 built houses and planted fields around the banks in order that they might 

 enjoy the priceless gift of their most potent deity. 



In southwestern Arizona the sheetflood is well known to the Indians 

 and to those Mexican and American rancheros who chance to be favor- 

 ably situated, and they are well and expensively known to railway corpo- 

 rations, who sometimes have five miles or more of track washed out by 

 a single storm perhaps sweeping over a smooth plain without a single 

 waterway before, and with only a few new-made gullies after, the catas- 

 trophe. In essential features the local lay testimony is everywhere alike ; 

 there is a storm with exceptional precipitation, the water simply floods 

 the surface in a muddy torrent or " wash " stretching as far as the eye 

 can reach, the ground is swept of loose debris, and even of surface sands, 

 while flotsam and sand heaps are piled up here and there ; and in a few 

 minutes, or perhaps a few hours in the lower valleys, the flood slackens 

 and almost immediately disappears. 



Such is the character of the sheetflood, as determined from direct ob- 

 servation, lay testimony, and the evidence of effects ; and were it need- 

 ful this evidence might be many times multiplied. 



Conditions requisite for sheetflooding : — The main (perhaps the sole) source 

 of the sheetflood is storm- water, comprising that shed from the moun- 

 tains and that falling directly on the intermontane plains : and since the 

 mountains are low and form only a small fraction of the surface, it seems 

 clear that the chief source is the storm of the plains. 



The first requisite for typical sheetflooding, then, is precipitation so 

 rapid as to exceed immediate absorption by the dry earth and immediate 

 evaporation in the drier air (for usually, in the Sonoran district, the pre- 

 cipitation horizon is some yards or hundreds of yards above ground, and 

 the lower strata of the air are so hot and dry as to take up much of the 

 falling and a part of the fallen moisture) ; and this involves several at- 

 tendant conditions : One condition is that the temperature shall be high 

 and the capacity for aqueous vapor great, in order that a large quantity 

 of water may be produced when precipitation occurs ; and this condi- 

 tion is amply met in the highly heated Sonoran district. An attendant 

 condition is that the precipitation shall be rapid ; and this also is met 

 by the subtropical climate and wind-disturbing topography of the dis- 



