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Introduction:
Managing an extreme cli
mate
words of
William Jennings, a prominent water
lawyer in the early 20th century, “It’s hard to
stop (the water) and there are very few
dam sites.”
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Together, the peculiar geology and hydrology
give this region the greatest variability in runoff
between the wettest and driest years of any-
where in the United States. At the low end,
runoff
may amount
to only five percent
of an average year,
while at the high
end, it can be
seven ti
mes more
than the average.
One of the driest
years on record
was 1899–1900,
when the El
Capitan dam site on the San Diego River
received only 980 acre-feet of runoff. The
same site received 200,400 acre-feet in
1915–1916, the year of the Hatfield flood
(Chapter 5). This extreme variability makes
storage reservoirs a necessity, yet it also
makes planning their capacity and building
flood-proof dams particularly challenging.
Historically, the storage requirements were
often underesti
mated; floods
broke dams all too frequently. To
make room for the occasional
flood, most reservoirs in the
county are sized so they are filled
to only about 40 percent of
capacity during normal years.
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To complicate matters, not all of
the rainfall
results in
runoff. If
the yearly
average of
10 inches
falls in two
or three
major
storms,
much of
the water runs into streams and
makes its way to reservoirs. If,
however, that 10 inches falls as fre-
quent sprinkles, which is often the case, it
seeps into the ground and evaporates without
producing any real runoff.
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In short, water is one of this region’s most
formidable challenges, requiring community
cooperation and engineering ingenuity.
San Diego’s seven principal rivers
Santa Margarita River
San Dieguito River
San Luis Rey River
San Diego River
Sweetwater River
Tijuana River
Otay River
AN ACRE-FOOT
Large amounts of
water are measured in acre-feet.
• 1 acre-foot = 325,900 gallons
• It covers a football field to the depth
of approxi
mately one foot.
• It is the amount of
water used by two typical
households in a year.
• An average person uses 144 gallons per day,
or about one-eighth of an acre-foot per year.