WORK OF WATER IN ARID REGIONS 578 



To the floodsheet has been ascribed the chief planorasive effects of the 

 arid regions. The law of running waters in the desert has been thought 

 to be contrary to what it is in a humid climate. Instead of surface 

 waters gathering into streams, they have been regarded as spreading out 

 into sheets. Mainly to this peculiarity of sheet-water behavior the level 

 features of the arid landscape have been thought to be due. That these 

 deductions are not entirely warranted by close observation of plains sheet- 

 flood effects is conclusively shown by a number of facts. 



The real reason why excessive desert waters flow down the inclined 

 plains surface in broad sheets — floodsheets — rather than in na"^row chan- 

 nelways, as in a normal humid country, is that the plains are already 

 prepared as such for the flood waters. The plains were there before the 

 waters came. The moving waters do not form the plains. The cor- 

 rading effects of running waters are the same in the desert country as 

 they are in the most humid land. The main difference lies in the fact 

 that copious rainfall in the desert is far less frequent than in the humid 

 region. On an average, a given locality probably does not have suffi- 

 ciently heavy precipitation to form a floodsheet oftener than once in a 

 dozen years. The gradients of the intermont plains are all ample for 

 very effective work by water. The slopes from the mountain bases have 

 usually at least a 2 or 3 per cent grade, and often very much higher 

 slopes — 150 to 200 feet to the mile. Besides, the middle of the plains 

 have a pitch nearly as high. N'evertheless the plains surface remains 

 uncorraded by the sporadic waters, for the reason that ^^between showers" 

 all inroads of normal water action on the plains surface are quickly filled 

 up and smoothed over by the drifting, wind-blown soils. A freshet gully 

 may last a day or a week, and is then smoothed over and obliterated. It 

 is a thousand weeks before another may be formed in the neighborhood. 

 To the casual observer, water action on the desert is not normal, because 

 its corrading effects are immediately and completely counteracted by the 

 more powerful and constant wind effects. In an upraised region the nor- 

 mal effects of running water is to excavate trenches, ravines, and valleys ; 

 the tendency of the winds, when they can act, is to smooth over inequali- 

 ties in the surface. 



Were the leveling tendencies of the winds wholly absent from the desert 

 region, it is quite probable that the corrasion effects of what surface 

 waters there are would be much the same as they are in the humid lands, 

 differing only in degree. This is well shown in cases where wing dams 

 have been constructed to protect lines of railway from the disasters of the 

 floodsheet, and the latter has come before the earthworks have had time 

 to be leveled by the winds. In one instance in particular the culvert and 



