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sloping l oodplains of the Central Valley. In those mountains, only the larger
l ood deposits are preserved, in contrast to the sequence of l ood deposits that
built up over time in the Sacramento Valley. h is is due to the erosive power of
these l oods in the narrow canyons that destroys any evidence of older, smaller
l oods. Researchers at the U.S. Geological Survey discovered two enormous
l ood deposits preserved in this region, estimated to be larger than one of the
most catastrophic historical l oods in Northern California, which occurred in
1964. h ese two l oods were dated by counting the growth rings of the oldest
trees rooted in the l ood deposits, and they occurred in approximately AD 1600
and 1750. A l ood layer dated between AD 1750 and 1770 was also found in the
Sacramento Valley by Byrne and Sullivan, and the AD 1600 l ood deposit was
likely the same as the AD 1605 events found in both the Sacramento Valley
and the Santa Barbara Basin. h e evidence for a 200-year recurrence interval
for l oods as large or larger than the 1861-62 event in California is growing,
bringing more urgency to l ood policy and planning ef orts in the state.
the ad 1605 megaflood
h e evidence for a megal ood occurring in AD 1605 has been found through-
out California, including in the northwest, central, and southern regions
as well as in San Francisco Bay, making this a truly extraordinary event
even by “megal ood” standards. In the Little Packer Lake record from the
Sacramento River basin, this l ood was estimated to be at least 50 percent
greater than any of the four 200-year events that Sullivan and Byrne detected
in their cores. It must have been a catastrophe of gargantuan proportions,
clearing hill slopes, drowning the Central Valley, and inundating most of the
state before ultimately l owing through the Sacramento-San Joaquin Delta,
through the San Francisco Bay, and out to the Pacii c Ocean.
Sediment cores taken from beneath the southern San Francisco Bay
contain oxygen isotope evidence that this time period was marked by very
low salinity in the estuary, presumably from the high river inl ows from the
watershed. In addition, cores taken from the northern reach of the bay, closer
to where rivers l ow in through the delta, contain a sedimentary hiatus, sug-
gesting that the l ood was large enough to erode and wash away the sediments
out to the Pacii c Ocean.
Intrigued by the phenomenon of cataclysmic l oods and their pos-
sible causes, Schimmelmann combed through the scientii c literature and
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