Geoscience Reference
In-Depth Information
Piles of belongings were found, apparently let behind as the people aban-
doned their settlements and l ed, some to live in fortii ed hideouts carved in
the clif faces, protecting their hoarded food from enemies. h e population
of the region is known to have plummeted by 85 percent in a short span of
time, but the causes of the collapse are still debated. Many make the case
that climate was clearly a factor. In this chapter, we explore some of the
climate evidence for drought, beginning with marshes along the California
coast.
drought evidence
San Francisco Bay
Vast tidal marshlands have grown up around the edges of San Francisco Bay
for roughly 6,000 years, at er sea levels began to stabilize. h e slower rate of
sea-level rise allowed marshes to establish themselves and keep pace with the
encroaching water. As the marshes continued to grow, their annual remains
accumulated, decomposing slowly in the anaerobic conditions below the
surface and forming peat. h ese marsh peats are important climate archives
for scientists, safeguarding information about climatic and environmental
conditions over the millennia.
h e San Francisco Bay is like a “funnel” for water draining Northern and
Central California out to the Pacii c Ocean. h e bay and its watershed can
be seen in dramatic clarity in satellite images, which show a web of river
tributaries from the Coast Ranges and the Sierra Nevada to the Central
Valley toward the bay. Fit y miles inland from the Golden Gate, these
rivers converge to form the Sacramento-San Joaquin Delta, where the waters
mingle and l ow through the bay and out Golden Gate to the Pacii c. Within
San Francisco Bay, fresh river water meets and mixes with the incoming
ocean water, producing a range of salinity: fresh in the delta, saline in the
central part of the bay near the Golden Gate, and brackish (intermediate)
in between.
Brackish waters provide unique habitats for plants growing in adjacent
tidal marshlands, creating more diverse plant communities than either the
salt marshes near the Pacii c Ocean or the freshwater marshes of the delta.
h e plants and organisms living in these tidal marshes must be adaptable: the
daily tides bring in not only more water but more
salty
water. Furthermore,
