Geoscience Reference
In-Depth Information
wet season, dumping rain on the coastal settlements and throughout the
Central Valley and the lower mountain slopes. Winters were bringing more
storms that were colder, and in the Sierra Nevada the snowpack grew thicker
and lower on the mountain slopes than it had been in human memory. h is
period was known as the Little Ice Age, and, elsewhere around the globe,
rivers and canals froze throughout Europe, and valley glaciers grew larger in
northern and central Europe, New Zealand, and Alaska. h e h ames River
in London froze at least eleven times in the seventeenth century. Lower
summer temperatures throughout Europe decreased the growing season by
several weeks, causing crop failures in Scotland, Norway, and Switzerland
and leading to widespread famine. In Norway, there is evidence for more
frequent landslides, avalanches, and l oods, and, in the Alps, expanding gla-
ciers occasionally crossed valley l oors, damming streams and forming glacial
lakes. h ese lakes would on occasion break through the ice dams holding
them back, l ooding the valleys below.
In California, some of the storms originated in tropical regions of the
Pacii c, delivering copious amounts of warm rain that led to l ooding. h ese
storms would have swept over the Coast Ranges, across the Central Valley,
and up into the Sierra Nevada, delivering heavy rain. h e rain and the
rapidly melting snow would have caused small creeks and rivers to swell into
raging torrents, ripping up vegetation and soils in the mountains, and turn-
ing California's Central Valley into a vast inland sea.
Many proxy climate records from throughout California contain evidence
that such extreme winter l oods occurred between 900 and 150 years ago.
Tidal marshes around the San Francisco Bay contain buried evidence of
these events in their sediments. Normally, the inl owing river waters spread
across the marshes, depositing only a small amount of the i nest sediments—
clays and silts. Floodwaters, however, carry larger particles in their higher
energy l ows, depositing a layer of sand and silt on the marshes. Marsh sedi-
ment cores reveal that one such layer was deposited on the marshes around
AD 1100, and two others around AD 1400 and AD 1650.
Sediment cores taken from beneath the San Francisco Bay itself provide
more evidence of the l ooding: a gap or hiatus in the sedimentary sequence
suggests erosion of hundreds of years' worth of accumulated sediments from
the bay l oor. h e date for this hiatus matches the earliest of the sand layers
in the marsh cores. h at such an event is recorded in San Francisco Bay estua-
rine and marsh sediments suggests a regionally signii cant l ood, af ecting
almost half of the state—the drainage area of the bay.
