100 W J MCGEE — SHEETFLOOD EROSION. 



direct observation, from indirect observation on flotsam, and from con- 

 sistent lay testimony. 



During the 1894 expedition a moderate local rain occurred while the 

 party were at a Papago rancheria near Rancho de Bosque, some 15 miles 

 north of the international boundary at Nogales ; the rainfall was perhaps 

 one-fifth of an inch, sufficient to moisten the dry ground and saturate 

 clothing despite the concurrent evaporation, and was probably greater 

 in the adjacent foothills of Santa Rita range. The road was sensibly 

 level, having only the 20-feet-to-the-mile grade of Santa Cruz valley ; 

 it ran across the much stronger slope from the range toward the river, 

 and an arroya embouched from low terraciform foothills not more than 

 200 or 300 yards up the slope. Thus the arroya opened not on a per- 

 ceptible fan but on a sensibly uniform plain of sand and silt with occa- 

 sional pebbles sloping perhaps a 150 feet lo the mile. The shower passed 

 in a few minutes and the sun reappeared, rapidly drying the ground to 

 whiteness. Within half an hour a roar was heard in the foothills, rap- 

 idly increasing in volume ; the teamster was startled, and set out along 

 the road up the valley at best speed; but before he had gone 100 yards 

 the flood was about him. The water was thick with mud, slimy w 7 ith 

 foam, and loaded with twigs, dead leaflets and other flotsam ; it was seen 

 up and down the road several hundred yards in either direction or fully 

 half a mile in all, covering the entire surface on both sides of the road, 

 save a few islands protected by exceptionally large mesquite clumps at 

 their upper ends. The torrent advanced at race-horse speed at first, but, 

 slowing rapidly, died out in irregular lobes not more than a quarter of a 

 mile below the road ; yet, though so broad and tumultuous, it was no- 

 where more than about 18 inches and generally only 8 to 12 inches in 

 depth, the diminution in depth in the direction of flow being less rapid 

 than the diminution in velochty. The front of the flood was commonly 

 a low, lobate wall of water 6 to 12 inches high, sloping backward where 

 the flow was obstructed by shrubbery, but in the open curling over and 

 breaking in a belt of foam like the surf on a beach; and it was evident 

 that most of the water first touching the earth as the wave advanced was 

 immediately absorbed and as quickly replaced by the on-coming torrent 

 rushing over previously wetted ground. Within the flood, transverse 

 waves arose constantly, forming breakers with such frequency as to churn 

 the mud-laden torrent into mud-tinted foam ; and even when breakers 

 were not formed it was evident that the viscid mass rolled rather than 

 slid down the diminishing slope, with diminishing vigor despite the con- 

 stant renewal from the rear. Such w r ere the conspicuous features of the 

 sheetflood — a thick film of muddy slime rolling viscously over a gently- 

 sloping plain ; and this film was a transformed stream still roaring 



