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EPHEMERAL STREAMS 33 '- '''0- "".... -4'$ '- .......... "by ,..... ~o"........ 100 90 80 1;; ~ . 70 " ~ ~ > 60 " < ~ iD " < 50 w ~ m < z 0 40 ~ w 30 20 100 fanwilh nocl1l"oel Olle shallo~ channel 500 1000 'i'lIIIIIIV//II&. GENERALIZED CROSS SECTION IN GULLIED REACH r," -- shalkM no c~annel chll'l!Iel - 17$.0 /;/'/,i'$/,i'@l' /@/,i',?' GUlly bed ~ 1500 DISTANCE. IN FEET 2000 2500 3000 FIQURE 27.-Profilo of a discontinuous gullY, Arroyo Falta, near Las Dos, 6 miles northwest of Santo. Fe, N. tIex. To extend the analogy between field conditions and the postulated stages of development, it was noted that another gully nearby and parallel to the one shown in figurc 27 could be characterized by diagram 5 (fig, 26) in that the depth remained nearly uniform and width had developed to a point where small patches of flood plain were beginning to appear within the gully walls. So it appeal'S that the discontinuous gully changes progressively through a series of stages to be trans- formed into a continuous gully of nearly uniform depth, The early stages are characterized by relatively flat slope of the discontinuous gully bed, but slope gradually increllSes with increasing channel width until the slope becomes nearly equal to that of the origin al ungullied valley floor. The relation between the hydraulic variables and the tendency for establishment and maintenance of a quasi-equilibrium, even in headwater ephemeral chan- nels, provide a logical explanation for the development of continuous gullies from discontinuous ones. With- out the hydraulic argument, the transition from discon- tinuous to continuous channels could not be explained, SUMMARY The brpad, sandy, and usually dry washes, and the trenchlike arroyos of the semiarid Southwest appear to be the antithesis of the perennial rivers of the humid East. Probably the most striking fact which emerges with the collection and analysis of hydraulic data per- taining to these apparently different channel systems is the similarity between them. Differing in detail, to be sure, the interrelations among hydraulic and physio- graphic factors in ephemeral and perennial channel systems are generally similar. Differences observed are in the direction which might be expected from the hydrologic and physiographic character of the areas, and for this rellSon the observed differences in values of hydraulic factors can be assumed valid. The same forces or processes operating in a river channel which lead to the condition of grade also exist in channels which are not graded, This leads to a tendency for adjustment among hydraulic parameters toward a condition of quasi-equilibrium between trans- / porting capability and load delivered to the channel / system. Even when complete adjustment is not/ achieved, the disequilibrium is spread through relatively long reaches of channel. Furthermore, when one of the ordinarily adjustable hydraulic factors is fixed or invariant, the other factors mutually adapt toward quasi-equilibrium under the prevailing conditions. The ephemeral streams studied provide examples of each of these general situations. These channels are probably downcutting slowly, for they are located in the headwater parts of their drainage areas where over