Texas Heritage, Volume 18, Number 3, Summer 2000 Page: 25
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pographic break alone is sufficient to slow
the movement of a tropical air mass and to
trigger spectacular convection rainstorms
when moisture-laden air rises and cools as
it flows up the escarpment. Convective
storms stimulated by a topographic feature
are known as orographic storms.
Orographic lift has contributed to many
of the historical rains and floods along the
southern margin of the Edwards Plateau.
But another phenomenon figures prominently
in a majority of the historical rain
events. In spring and fall, tropical air masses
moving northward from the Gulf often
encounter fronts along the leading edge of
southward-moving masses of cooler, drier
air. Anywhere such contrasting air masses
collide, rainstorms are almost certain to
form, but the results are even more dramatic
along the Balcones Escarpment with
the addition of its orographic effect. Slowing
of tropical air masses by the escarpment
increases the chances that contact with a
cold front will occur close to the escarpment.
Thus it is that physiography and climate
combine in Central and Western
Texas to produce extraordinarily heavy
rainfall events. As if that were not enough,
those same elements of physiography and
climate impart further conditions on the
area that lead to disastrous consequences
of the heavy rains.
Streams flowing southward and southeastward
across the Edwards Plateau have
cut steep-gradient, narrow canyons through
the limestone bedrock of the plateau. Soils
on the plateau surface and slopes are thin
to absent and, under the prevailing conditions
of limited rainfall, vegetation cover
is too sparse to retain rainwater. The combination
of steep gradients, thin soils, and
sparse vegetation results in rapid runoff of
rainwater and the tendency to generate
flash floods at times of heavy rains.
With its dams and other flood control
installations, the main stream of the Rio
Grande will flood at times, but risk to life
and property is no longer great. The smaller
rivers, creeks, and draws that descend from
the uplands along the margins of the Rio
Grande valley, on the other hand, pose great
risks of flash floods. The communities located
along these streams are generally unprepared
for the flooding that will result from
storms that can dump several times the annual
average rainfall in a day or two.
The 1954 flood of the Rio Grande affected
downstream communities such as Eagle Pass
and Piedras Negras, shown above. Courtesy
of Al Kinsall.
Dr. Michael B. Collins is research associate
at the Texas Archeological Research Laboratory
Author's acknowledgments: I thank Jack
Skiles and the late Guy Skiles of Langtry
and the late Perry Brotherton of Val Verde
County for sharing their recollections of
the 1954 Pecos River flood. Also, thanks
to Sharon Dornheim for searching the
newspaper archives at the University of
Texas at Austin.
HERITAGE * 25 * SUMMER 2000
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Texas Historical Foundation. Texas Heritage, Volume 18, Number 3, Summer 2000, periodical, Summer 2000; Austin, Texas. (texashistory.unt.edu/ark:/67531/metapth45389/m1/25/: accessed July 23, 2017), University of North Texas Libraries, The Portal to Texas History, texashistory.unt.edu; crediting Texas Historical Foundation.