The San Pedro River: A Long View

  Dear HCN,


The article on competing water usages for Sierra Vista, Fort Huachuca, and the San Pedro Riparian National Conservation Area opens the door for more general consideration of the dramatic geologic and ecologic changes that have affected the San Pedro River over the past century (HCN, 6/12/95). The paired "before" and "after" pictures (pages 10 and 11), showing cattle on barren intrachannel sandbars a decade ago and luxuriant shrubbery within the river channel today, is a good point of departure for thinking about a longer time frame of river history. For example, labeling the picture of luxuriant current vegetation within and along the river channel as "after recovery" is perhaps a misnomer, for such growth along the river has probably never before existed, at least not within the past few centuries. Let me explain.


The story of the recent natural history of the San Pedro River is well told by Richard Hereford in 1993 Geological Society of America Special Paper 282, Entrenchment and Widening of the Upper San Pedro River, Arizona (46 pages, available from GSA, Box 9140, Boulder, CO 80301 for $15 plus sales tax for Colorado residents). The modern San Pedro channel is incised into its former floodplain between cutbanks that range from a few feet to a few tens of feet in height. The entrenchment of the channel evidently occurred in rapid stages of erosion (-arroyo cutting') during the interval 1890-1908.


Prior to the arroyo cutting near the turn of the century, the San Pedro River occupied a shallow channel flowing down a partly marshy and generally treeless but grassy floodplain. The old floodplain is now stranded, at elevations above the modern channel, as stream terraces covered by dense mesquite thickets (-bosques'). In the middle of the last century, the Mormon Battalion of Mexican War fame drove its wagons down the valley without hindrance by either thickets or arroyos.


For several decades after initial entrenchment, the arroyo of the San Pedro channel continued to widen itself by lateral bank erosion. Consequently, its banks were too unstable to allow riparian trees to become established until about the middle of the present century. Photographs taken in the decade of 1930-1940 show only low shrubs along the channel banks. The present growth of stately riparian trees forming a tall gallery forest dates back only 40-50 years, with 1955 as the probable earliest date for a fully developed riparian corridor.


In a literal sense, we are thus now engaged in a fascinating ecological experiment with the establishment of a riparian conservation area. By banning grazing along the San Pedro, we are gambling that the riparian forest will be as content with the absence of cattle as it was with their presence during the past half-century as it came into being. The effect of augmented undergrowth on the health of the big trees should probably be monitored closely.


San Pedro affairs have rather broad significance, for many drainage systems in the southwestern states and adjacent parts of Mexico experienced analogous arroyo cutting or entrenchment a decade or so before and/or after 1900. This regional episode of channel erosion is commonly ascribed in the popular press to the effects of overgrazing, but geoscientists have repeatedly concluded over the past half century that it was probably caused fundamentally by a climatic shift that changed patterns of rainfall intensity, and/or the spacing of rainfall through the months of a year.


Part of the reason for fingering subtle climate change, instead of grazing, is knowledge that analogous episodes of prehistoric arroyo cutting, separated by intervals of backfilling, occurred prior to the arrival of any domestic livestock on the scene. Along the San Pedro, for example, the alluvian into which the present arroyo was cut around 1900 was apparently deposited mainly during the interval 1450-1850. That period of sedimentation evidently filled and ultimately erased arroyos that existed prior to 1450.


These perspectives are offered in thumbnail fashion here in hopes they will stimulate wider interest in paleo- landscape analysis. In my judgment, there is no area of research more important for environmental concerns. Unless we can understand fully how we got where we are, we have little hope of planning the future with any success.





William R. Dickinson


Tucson, Arizona





William Dickinson is past president of the Geological Society of America and emeritus professor of geosciences at the University of Arizona.








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