Geoscience Reference
In-Depth Information
the bottom of the estuary while the i ner, lighter ones are washed onto the
surrounding marshes, forming steadily accumulating layers.
h e Sacramento River drains the northern part of the watershed, carrying
sediments from the northern Sierra Nevada and southern Cascades, and the
San Joaquin River drains the southern part of the watershed. h e mineral
composition of the sediments is as valuable to paleoclimatologists as i nger-
prints are to police detectives, providing evidence of origin. h e changing
proportions of sediments from the northern and southern parts of the water-
shed provide details of how climate in Northern and Southern California has
varied over time, and these changes have shown that the Medieval droughts
were indeed felt throughout the watershed of the estuary, in both northern
and southern portions—but not at the same time.
During the i rst of the megadroughts, which began around AD 900, the
southern Sierra Nevada experienced the initial severe conditions. Run-of
l ows were so low that almost no sediments from that region reached the
bay. In the northern portion of the watershed, conditions remained normal
at i rst, and the Sacramento River l ows continued to transport sediments.
However, as the drought persisted, it spread northward, causing reduced
l ows of the Sacramento River and therefore bringing less sediment from that
region to the bay. At the same time, l ows from the San Joaquin River, drain-
ing the southern watershed, increased somewhat (around AD 1080). h e
second major drought (between AD 1200 and 1400) shows a similar pattern.
When we compare these results with tree-ring studies from the northern
Sacramento River watershed and the southern San Joaquin River watershed,
we see agreement: the droughts across California did not always occur at
the same time, but alternated between the northern half and the southern
half of the state. h e Medieval Climate Anomaly was a period of unusual
climate, but the impacts on precipitation over the American West were not
uniform in magnitude, duration, or geographic location. What could have
caused these unusually dry periods in the West?
clues from the pacific ocean
h e search for the cause of this period of variability and climate extremes led
researchers to turn their attention to the primary source of the American
West's climate: the Pacii c Ocean. Some of the i rst clues emerged from the
ocean just of coastal Southern California.
