26 THE SERI INDIANS [eth.ann.17 



the <liscliarge from the waterways of adjacent sierras and buttes; they 

 are commonly miles and freqnently dozens or scores of miles in width, 

 and the linear flow may range from a fraction of a mile to scores of 

 miles according to the heaviness of the rainfall and the consequent 

 dilution of the mud. Such slieetHoods, especially those produced by 

 considerable rains, arc characteristic agents of erosion throughout most 

 of the province; their teudeucy is to aggrade depressions and corrade 

 laterally, and thus to produce smooth plains of gentle slope interrupted 

 only by exceptionally precipitous and rugged mountain remnants. A 

 part of the sheetflood water joins the stronger mountain-born streams, 

 particularly toward the end of the great storm whereby earth and air 

 are saturated; another ])art forms ground-water, which slowly linds its 

 way down the slopes toward the principal valleys, jjcrhaps to rea])pear 

 as springs or to sup[)ly wells. These with certain other conditions 

 determine the water supply available for habitation throughout Seri- 

 land and adjacent Papagueria. 



Another condition of prime importance arises in a secular tilting of 

 the entire province southwestward. This tilting is connected with the 

 upthrust of the Sierra .Aladre and the uplifting of the plateau (iountry 

 and the southern liocky mountain region north of the international 

 boundary. Its rate is measured by the erosion of the Grand Canyon of 

 the Colorado and other gorges; aiul its dates, in terms of the geologic 

 time-scale, run at least from the middle Tertiary to the present, or 

 throughout the Ifeocene and Pleistocene. Throughout this vast period 

 the ettect of the tilting in the Sonorau province has been to invigorate 

 streams flowing southward, and to paralyze streams flowing toward 

 the northerly and easterly compass points; accordingly the streams 

 flowing toward the gulf have eroded their channels eftectively during 

 the ages, and have frequently retrogressed entirely through outlying 

 ranges: so that throughout the province the divides seldom correspond 

 with the sierra crests. 



A typical stream of the province is Rio Bacuache, one of the two 

 practicable overland ways into Seriland (albeit never surveyed until 

 traversed by the 1895 expedition). Viewed in its simple geographic 

 aspect, this stream may be said to originate in a broad valley parallel 

 with the gulf and the high sierra, 200 miles northeast of Kino bay: its 

 halfdozen tributary arroyos (sun-baked sand-washes during three 

 hundred and sixty days and nmd-torrents during live days of the 

 average year) gather in the sheetflood plain and unite at Pozo 

 Noriega, where the ground-water gives permanent supply to a well ; then 

 the channel cleaves a rocky sierra 3,000 feet high in a narrow gorge, 

 and within this canyon the ground-water gathered in the valley above 

 seeps to the surface of the sand wash and flows in a practically perma- 

 nent streamlet throughout the 4 or 5 miles forming the width of the 

 sierra; then the liquid sinks, and 25 miles of blistering sand- wash 

 (interrupted by a single lateral spring) stretch across the next valley 



