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<br /> <br />EPHEMERAL STREAMS <br /> <br />changing slope through long reaches of river, slope <br />adjusts only slowly. <br />Analysis of river data showed that at a given dis- <br />charge, the same suspended-sediment load will be <br />can'ied by a narrow, deep channel at a particular <br />velocity as in a 'Wider, shallower channel at some","'hat <br />higher velocity (see fig. 15, p, 23, in Leopold and <br />.\laddock, 1953). The early stage of a discontinuous <br />gully is considered to represent the narrow, deep con- <br />ditio)l, and at a later stage, lateral cutting and slumping <br />of the arroyo walls lead to widening, with a consequent <br />shallowing of the cross section of flowing water at a <br />purt,icular discharge. K ow, under conditions of a given <br /> <br />31 <br /> <br />and constant roughness, a relatively large depth and <br />small velocity requires a small value of slope; that is, <br />a flat gradient, as can be seen by considering the <br />.\fanning-type equation <br /> <br />dl"l <br />"=1.5 ----,- <br />n <br /> <br />As widening progresses during a later stage in gully <br />development, to carry a particular sediment load at a <br />given discharge requires a slightly increased velocity <br />as the width-depth ratio increases. To achieve the <br />larger velocity with decreasing depth at a given rough- <br />ness means an increase in the value of slope. <br /> <br /> <br /> <br />Initi!1 discan~nUDUS guliies <br /> <br /> <br /> <br /> <br />Discontinuous Ilullin ad.ance <br /> <br />Sligeotiirstcoalescenu <br /> <br />Com~lete intei~~on into a lingle i:~lIy <br /> <br />LONGITUDINAL SECTIONS <br /> <br />A A' <br />8 r~lll$/jI l <br />A A' <br />-_C21//0"/11;1~ l <br />(2) 'rlllllllll;14,J A' <br />I <br />A A' <br />o FPII//IU I <br /> <br /> <br />8Ft <br /> <br />A' <br /> <br />.Jl <br /> <br />CROSS SECTIONS THROUGH A-A' <br /> <br />FIGt:'RE 2d.-Stages of development of an arroyo from discontinuous gullies. <br />