Geoscience Reference
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that Colorado River l ows experienced major l uctuations during that period.
Looking at reconstructed 25-year average l ow conditions, researchers have
found that it was a time of generally cooler conditions, with at least six wet
periods marked by signii cantly high l ows. However, there were also periods of
signii cantly low river l ows, including AD 1564-1600, AD 1844-48, and AD
1868-92. Over the past 450 years, the long-term annual average l ow for the
Colorado River was 14.6 million acre-feet. Flows during the identii ed dry peri-
ods were as low as 9.6 million acre-feet, only 66 percent of the long-term average.
Although these l ows were low, they were not as low as during the
Medieval Climate Anomaly. In particular, from AD 1130 to 1154, Colorado
River l ows were 85 percent lower than the twentieth-century mean. We will
discuss this variability and its origins further in the next chapter.
the dry-wet climate knockout
Some of the paleoclimate records reveal repeated patterns of dry and wet
climate extremes, such as severe droughts immediately followed by cata-
strophic l oods—or the opposite, wet periods followed by droughts. h ese
combinations can wreak havoc on ecosystems, especially when accompanied
by unusually large wildi res, soil erosion, and disease.
One example of a “dry-wet knockout” has been documented by Scott
Mensing, Roger Byrne, and Joel Michaelsen, who analyzed the l ux of large
charcoal particles and pollen deposited in sediments cored from the Santa
Barbara Basin in Southern California. h eir analysis reveals that the larg-
est i res—those ot en associated with warm, dry Santa Ana winds blowing
from the northeast to the Pacii c during the fall months—have repeatedly
occurred in the i rst year of a drought that follows an extended wet period.
Moreover, Mensing and his colleagues showed that between twenty and
thirty large wildi res occurred over the past 560 years, the majority following
unusually wet periods.
Examples of this pattern in the twentieth century include the wildi res
of 1955 and 1964, which followed two of the wettest winters of the century.
Investigators reason that vegetation grew prolii cally during years with greater
precipitation. When followed by unusually warm and dry summer conditions,
the additional vegetation provided extra fuel for unusually large forest i res.
Periods of deep drought that are followed by heavy rain and l ooding
can also cause problems. h
is phenomenon has been observed both in the
