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Braiding in Semiarid Sand-Bed Streams Robert B. Howard* Various causes for braided (also termed anastomosing, bifurcating , or multiple-channel) streams have been postulated, but there remains considerable confusion between factors which instigate braiding and factors which enhance it. Further, there has been little previous effort to match causal factors with specific climatic environments to permit a fuller explanation of the braiding phenomenon . Braided streams are characteristic of arid, semiarid, and pro-glacial environments and they may also occur in some environments not produced by contemporary climate at all. Therefore, one should not expect to find a single mechanism that causes braiding under all environmental circumstances. For the present study, braided sand-bed streams1 located in semiarid sectors of Wyoming, Utah, and southern California were measured for their braiding geometry, sediment characteristics, streamflow, and channel morphology. The resulting field data were statistically analyzed by non-parametric correlation methods to find relationships among materials, processes, and braided forms. The results of the analyses, coupled with additional information from the literature, have provided a basis for evaluating the various causes of braiding cited by other researchers. These citations are next considered briefly by way of affording background for the present study. * Dr. Howard is Assistant Professor of Geography at California State University , Northridge 91324. 1 The sand-bed condition was imposed to eliminate the difficulty of working with streams whose sediment is primarily gravel. Sand-sized particles have the lowest entrainment velocities and therefore are fairly mobile, whereas larger particles are generally immobile under discharges less than bankfull. 51 52ASSOCIATION OF PACIFIC COAST GEOGRAPHERS Hypothetical Causes of Braiding Six of the numerous causes for braiding advanced in the literature over the years seem particularly prominent and therefore worthy of review, although at the outset some appear to be contributing , rather than initiating, factors. Extended discussion of these causes will be made in subsequent sections of this article. Aggradation is one of the oldest presumed causes for braiding. This condition was used by Shen and Vedula in a successfully tested model based on the principle of diminishing boundary shear stress with time.2 Their aggrading flume channel, in fact, developed a braided pattern only a few hours after the experiment was initiated. Large particle sizes, coupled with Quaternary geomorphic history , have been proposed to explain extensive braided reaches of the Mississippi River, where bars or islands are adjacent to tributaries, suggesting that debris transported by tributary streams is too coarse for the master stream to remove.3 Discharge fluctuation has been suggested by Doeglas as a cause of braiding in streams of southern France, the fluctuations in discharge being attributed to the highly variable rainfall in that region.4 In addition, slight discharge fluctuations upon the extremely uneven floodplain of a braided stream have been shown through time-lapse motion pictures to produce islands and channels in a meltwater stream at the snout of the Athabaska Glacier in Jasper National Park, Canada.5 However, evidence contrary to the importance of discharge fluctuation in the initiation of braiding has been cited by Leopold, Wolman, and Miller.6 These authors use as an example a stream in Wyoming which has been found to change abruptly from 2 H. W. Shen and S. Vedula, "A Basic Cause of a Braided Channel," Proc. 13th Congr. Intern. Assoc. Hydr. Res., Kyoto, Vol. 5-1 (1969). 3 W. W. Rubey, "Geology and Mineral Resources of the Hardin and Brussels Quadrangles (in Illinois)," U.S. Geol. Survey Prof. Paper 218 (1952), pp. 123-124. 4 D. J. Doeglas, "Structure of Braided River Deposits," Sedimentology, Vol. 1 (1962), pp. 167-190. 5R. Kucera, Glacier on the Move (New York: Encyclopedia Britannica Films, 1973). ß L. B. Leopold, M. G. Wolman, and J. P. Miller, Fluvial Processes in Geomorphohgy (San Francisco: W. H. Freeman, 1964), p. 294. YEARBOOK · VOLUME 39 · 197753 meandering to braided in the absence of local tributaries and without variation in water discharge or sediment load between the twohabits. This condition suggests that discharge fluctuation, if at all influential , operates only to enhance braiding. Heterogeneous (unsorted) bed material and its connection with the braiding process is cited by Leopold and others who note, however , that heterogeneity is likely to be...

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