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<br />
<br />PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS
<br />
<br />EPHEMERAL STREAMS-HYDRAULIC FACTORS AND THEIR RELATION
<br />TO THE DRAINAGE NET
<br />
<br />By LUNA B, I,EOPOLD and JOHN p, MILLER
<br />
<br />ABSTRACT
<br />
<br />The hydraulic factors of width, depth, velocity, and suspended-
<br />sediment load of ephemeral streams near Santa Fe, N. Mex.,
<br />were measured during flood flow. Later, channel slope was
<br />meflosured. These flood~flow data, in conjunction with an analy-
<br />sis of drainage-basin configuration by the methods proposed by
<br />Horton, are used to determine the generalized interrelation of
<br />stream order and hydraulic variables. The method developed
<br />for determining this interrelation allows an integration of the
<br />geographic and physiographic chara.cteristics of a drainage basin
<br />with the channel characteristics; specifically, the interrelation
<br />of the length, number, and drainage area of streams of various
<br />sizes with their respective discharge, width, depth, velocity,
<br />slope, channel roughness, and suspended-sediment load.
<br />These interrelations show that stream order is related to stream
<br />length, number of streams, drainage area, and discharge by
<br />simple exponential functions. The relation of discharge to
<br />width, depth, velocity, slope, and other hydraulic factors can be
<br />approximated by simple power functions. Thus, any pair of
<br />these factors is related by exponential or power functions.
<br />The data indicate that suspended-load measurements made
<br />during various stages of a few individual floods provide a close
<br />approximation to the suspended-load rating curve obtained from
<br />periodic measurements taken at a sediment station over a period
<br />of years.
<br />The analysis of the hydraulic data shows that in the ephemeral
<br />streams studied, velocity increases downstream at a faster rate
<br />than in perennial rivers. This appears to be associated with an
<br />increase in suspended-load concentration downstream in these
<br />ephemeral channels.
<br />The tendency for stream channels to maintain a. quasi-
<br />equilibrium with imposed discharge and load -is shown to be
<br />characteristic of ephemeral channem in the headwaters of the
<br />drainage ba.sin, even to the most headward rilL
<br />The formation of a discontinuous gully is analyzed in terms
<br />of this tendency. This type of gully is characterized by a
<br />gradient of the channel bed flatter than the gradient of the
<br />valley floor in which the gully is cut. The low gradient is
<br />explained as a hydraulic adaptation of channel slope to quasi-
<br />equilibrium "ith the narrow gully width. Examples indicate
<br />that as a discontinuous gully widens, its gradient steepens and
<br />its length consequently increases. Thus a. series of discon-
<br />tinuous gullies tends to coa.lesce into a continuous trench having
<br />a bed gradient nearly equal to the slope. of the original va.lley
<br />floor.
<br />
<br />INTRODUCTION AND ACKNOWLEDGMENTS
<br />
<br />The flow of water in natural channels may be
<br />described as perennial, intermittent, or ephemeral. A,
<br />perennial stream earries some flow at all times. An
<br />intermittent stream is one in which, at low flow, dry
<br />reaches alternate with flowing ones along the stream
<br />length. Those which carry water only during storms,
<br />and are therefore called ephemeral, are generally smaller
<br />but are mueh more numerous than perennial ones.
<br />From the divide to the mouth of a drainage basin, the
<br />inerease in channel size is accompanied by a decrease
<br />in the number of channels,
<br />Drainage channels are more apparent and more
<br />abundant in arid than in humid. regions where vegeta-
<br />tion hides small rills, Vegetation also tends to increase
<br />the length of overland flow, thereby decreasing channel
<br />density. For both kinds of areas, however, the land
<br />presents to the eye a contrast depending on the perspec-
<br />tive one assumes. In arid areas there is the panorama
<br />of broad, apparently smooth surfaces sweeping up to
<br />the abrupt fronts of mountain ranges or mesa escarp-
<br />ments. A closer examination may show that the whole
<br />countryside is actually cut into a myriad of rills, gullies,
<br />and arroyos. Humid regions, on the other hand, are
<br />typically characterized by rounded, rolling topography
<br />which is covered by natural or cultivated vegetation.
<br />Although many details of drainage nets are commonly
<br />concealed beneath vegetation, careful search discloses
<br />the presence of many more channels than can be in-
<br />ferred from a distant view,
<br />Despite the fact that the channels of ephemeral
<br />streams are generally recognized to have an important
<br />part in the erosion of the land and resultant production
<br />of fluvial landforms, they have not received careful or
<br />concentrated investigation. The present report is
<br />devoted to a preliminary quantitative description of
<br />flow characteristics, chaDnel properties, and configura-
<br />tion of the drainage net in arid regions only. However,
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