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<br />
<br />34
<br />
<br />PHYSIOGRAPHIC AND HYDRAULIC STUDIES OF RIVERS
<br />
<br />short periods geologically, land reduction is in progress,
<br />Thus the streams do not fulfill the requirement of
<br />stability which the term "grade" usually implies
<br />CW olman, 1954) . Yet, the channels are adjustable.
<br />These ephemeral streams in the Southwest are char-
<br />acterized by increasing concentration of suspended
<br />sediment in the dO\\~nstream direction, which results
<br />from loss of water by infiltration to the channel bed and
<br />to some lesser extent, from progressive pick-up of load
<br />from the stream bed. In response to this downstream
<br />change in sediment concentration, channel factors
<br />mutually adjust themselves toward quasi-equilibrium.
<br />One observable aspect of this adjustment is a greater
<br />downstream increase in velocity than is characteristic
<br />of normal perennial rivers.
<br />These same processes of adj ustment lead to a pro-
<br />gressively increasing width and decl'easing gr:adient
<br />downstream in n. nearly uniform manner, not only in
<br />the larger streams but headward as far as the upper-
<br />most rills. As an example of adaptation to a tempo-
<br />rarily fixed or predetermined value of one hydraulic
<br />factor, the width-slope relations in road-cut rills were
<br />investigated. It was shown that on a road-cut where
<br />slope was determined by a road grader rather than by
<br />natural processes, rills developed a width which fits
<br />the width-slope relations in natural ephemeral rills
<br />and channels in the same locality.
<br />The same general principles were used to analyze
<br />. the probable interrelation between hydraulic factors in
<br />gullies or discontinuous arroyos. This leads to the
<br />conclusion that the meehanics of gully development
<br />involve the same tendency for mutual adjustment
<br />of hydraulic faetors, even in a ehannel undergoing
<br />progressive erosion. The analysis suggests that
<br />measurements of flash flows in active gullies and
<br />suspended-sediment load would be pertinent, even
<br />
<br />essential, to an improved understanding of the manner
<br />and rate of gully erosion and effeetive measures for
<br />gully control.
<br />
<br />REFERENCES
<br />
<br />Babcock, H. Iv1., and Cushing, E. M., 1942, Recharge to ground-
<br />water from floods in a typical desert wash, Pinal County,
<br />Ariz.: Am. Geophys. Union Trans., pt. 1, p. 49-55.
<br />Bryan, Kirk, 1928, Historic evidence on changes in the channel of
<br />Rio Puerco, a tributary of the Rio Grande in K. )fex.:
<br />Jour. Geology, v. 36, p. 265-282.
<br />Gregg, J., 1844, Commerce of the prairies, New York, H. G.
<br />Langley Co., v. 2.
<br />Hack, J. T., 1956, Studies of longitudinal stream profiles in Virginia
<br />and :rvlaryland: U. S. GeoI. Survey Prof. Paper 294-B. (In
<br />preparation.)
<br />Horton, R. E., 1945, Erosional development. of streams and their
<br />drainage basins-hydrophysical approach to quantitative
<br />morphology: Geol. Soc. America, Bull. v. 56, p. 275-370.
<br />Jahns, R. H., 1949, Desert floods: Engr. and ScL 1.Tonthly,
<br />Pasadena, Calif., ~lay issue, p. 10--15.
<br />Leopold, Aida, 1921, A plea for recognition of artificial works
<br />in forest erosion control policy: Jour. Forestry, v. 19, no. 3.
<br />Leopold, L. B., 1942, Areal extent of intense rainfalls, New
<br />~:1exico and Arizona: Am. Geophys. Union Trans., pt. 2,
<br />p. 558-566,
<br />- 1953, Downstream change of velocity in rivers: Am. Jour.
<br />ScL, v. 251, p. 606-624.
<br />Leopold, L. B. and ~:laddock, Thomas, Jr., 1953, The hydraulic
<br />geometry of stream channels and some physiographic
<br />implications: U. S. GeoI. Survey Prof. Paper 252.
<br />Mackin) J. H., 1948, Concept of the graded river: GeoI. Soc.
<br />Amer. Bull. 59, p. 463-512.
<br />O'Brien, 1\-1. P., and Hickox, G. II., 1937, Applied fluid mechanics,
<br />Kew York, IvlcGraw-Hill Book Co., 360 p.
<br />Rubey, 'v. 'V., 1952, The geology and mineral resources of
<br />Hardin and Brussels quadrangles, Illinois: U. S. GeoI.
<br />Survey Prof. Paper 218.
<br />"Tolman, :\'.1. G., 1954a, The natural channel of Brandywine
<br />Creek, Pa., U. S. GeoI. Survey, Prof. Paper 271.
<br />- 1954b, A method of sampling coarse river-bed material:
<br />Am. Geophys. Union Trans.. v. 35, no. 6, p. 951-956.
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