SHEETFLOODING PREVAILS AT MIDHEIGHTS. 107 



trict; two-fifths or more of its area consists of vast torrential aprons 

 lightly veneered with detritus resting on baseleveled rocks, the inclina- 

 tion ranging between 75 and 250 feet per mile. 



The final requisite for sheetflooding is that the volume of water, the 

 mass of available debris for loading it, and the slope (and hence the 

 velocity) shall be so interrelated and balanced that every part of the sheet- 

 flood may be loaded to its capacity, and that any temporary or sporadic 

 increase in velocity may be quickly checked by overloading and conse- 

 quent reduction of velocity. The conditions affecting this requisite are 

 multifarious, probably beyond analysis, but it may be suggested that an 

 important — perhaps the essential — condition is a progressive paralysis or 

 weakening of the torrent due to the constant absorption and evaporation 

 of its liquid element, the solid element remaining to burden immediately 

 the falling or inflowing water. This condition is fully met in the parched 

 air and burning sands of Papagueria. 



On juxtaposing the requisite and qualifying conditions of sheetflood- 

 ing, they are found in harmony with the distinctive characteristics of 

 the greater part of the Sonoran district, and when the characteristics are 

 compared with the conditions observed in special localities the harmony 

 is rendered still more complete. It becomes evident that the sierras and 

 buttes lie outside the domain of sheetflooding, since their slopes are too 

 steep and their detritus too scanty ; that the mountain-born streams on 

 reaching the torrential plains must become quickly diverted and attenu- 

 ated into sheetfloods, provided their volume be sufficient; that the light 

 shower falling on the baselevel plain is absorbed, while the heavy shower 

 must spread into a moving film and the cloud-burst into an irresistible 

 sheetflood, sweeping all before it; and that the final feeble flow, whether 

 from distant barrancas or local seepage in depressions, must resume the 

 habit of the stream, pushing down toward the sea until the waters are 

 finally lost.* 



Erosive work of sheetfloods. — It may be affirmed from observation, both 

 direct and indirect, and from necessary inferences concerning land-forms 



* Reference is due to the work of Mr Willard D. Johnson during the second expedition, and to 

 his opinions concerning the somewhat anomalous topographic features of the Sonoran district. 

 Although sheetfloods were not witnessed during this expedition, Mr Johnson had opportunities 

 for studying flotsam records, and in the course of his admirable topographic surveys he was 

 much in contact with the features of the sand washes forming the fans or deltas of Rio Sonora, 

 Rio Bacuache, and other principal drainageways. His conception of the Sonoran flood was com- 

 monly expressed by the term "interlacing drainage," consisting of a multitude of broad, shal- 

 low, and swift streams, approximately parallel in course, constantly divaricating and reuniting 

 in such manner as to leave numerous islands, which were from time to time invaded and swept 

 away. This conception is undoubtedly accurate so far as the lower reaches of the waterways are 

 concerned, and it doubtless applies also to what the writer would consider as the lower or periph- 

 eral zone of sheetflooding proper, where there is a tendency toward the resumption of stream 

 habit. Unfortunately Mr Johnson's observations and conclusions are not yet published ; circum- 

 stances have prevented even the completion of his map, excepting the portion including Seriland ; 

 but important results may be anticipated. 



