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.
This article appeared in the print edition of the magazine with the headline The San Pedro River: A Long View.
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