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<br />390
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<br />JAMES E. DEACON AND W. L MINCKLEY
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<br />Fig, Ie, Rio los Positos, Cuatro Cienegas basin, Coahuila, Mexico. This stream,
<br />fed by a number of small springs, is about 1.5 meters across in the immediate fore-
<br />ground. Deposition of travertine and other salts along its channel precludes develop-
<br />ment of woody, riparian vegetation. Photograph by Charles Stone, August 1960.
<br />
<br />aquatic animals in places far downstream from upstream segments of per-
<br />ennial flow, but this occurs only when water percolating through coarse-
<br />grained sediments of the channel is forced to the surface by transverse
<br />deposits of impervious stone. Reaches such as this persist year-round,
<br />especially in canyons where shaded from direct, intense insolation, and they
<br />have long been important to the desert traveler-be it man, beast, fish,
<br />or fowl.
<br />Desert watercourses are plagued by extremes, and one of the most spec-
<br />tacular is the flash flood produced when local runoff concentrates into a
<br />relatively narrow channel. Conditions in a smaller creek, or even in a river,
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