Storm events and arroyo entrenchment

A comment by Mark A. Melton

I haven't read the paper by Hereford on entrenchment of
the upper San Pedro.  I have talked to eye witnesses who
told me that the entrenchment of the middle San Pedro
occurred in a single night in October, 1923, I believe,
when a Pacific tropical storm dumped some 6 inches of rain
on the San Pedro headwaters.  This is documented in
weather records both in Arizona and Sonora.  As there were
earlier such storms, 1919 I believe was one such, without
trenching, it seems fairly apparent that the governing
cause had to be overgrazing of the inner valley floor,
first, then overgrazing of the hillslopes leading to
increased runoff.

Mark A. Melton

U.S. Department of the Interior | U.S. Geological Survey
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