Geoscience Reference
In-Depth Information
past century and in the more distant past. Prolonged, multiyear droughts
followed by severe storms and massive l ooding are nature's version of a
one-two climate punch. University of Nevada, Reno, dendroclimatologist
Franco Biondi and his colleagues have studied these patterns and dubbed
them “dry-wet knockouts” because the l oodwaters ot en wash away hillsides
that have been denuded of vegetation during extended drought, transport-
ing huge volumes of sediment to coastal waters. h ere are many examples of
extended droughts—including those that occurred during Medieval times—
terminated by extremely wet years, which ot en lead to a chain-reaction of
impacts on local ecosystems.
During multiyear droughts, mountain forests become dangerously dry,
and summer lightning strikes spark wildi res that sweep through and engulf
the forests. Subsequent heavy rains generate l oods that can easily erode i re-
charred slopes, washing massive amounts of sediment and charcoal downhill,
eventually into streams and lakes. Once on the move, such sediments raise
the bed-level of the rivers, further adding to l ood potential. h ese sediment-
laden waters can disrupt aquatic ecosystems downstream in ponds, lakes,
estuaries, and even the coastal ocean.
Evidence of these past knockout l oods can be found throughout
the West. Tree stumps found in lakes in the southern and eastern Sierra
Nevada are what remain of medieval trees that grew for over a century dur-
ing the prolonged droughts of the Medieval Climate Anomaly and were
suddenly drowned when the drought was terminated by heavy rains and
huge l oods. Downstream in the San Francisco Bay tidal marshes, sedi-
ment cores reveal evidence of large l oods dating to around AD 1100, 1400,
and 1650. h ese appear to have occurred during the interval between the
Medieval megadroughts and immediately at er the Medieval droughts.
Similarly, in Yellowstone National Park, Wyoming, evidence of massive
debris l ows, caused by l ash l oods, has been dated to the end of the Medieval
drought.
h e greatest megal ood on record in the West over the past two
millennia—the AD 1605 event described above—also followed on the
heels of an extended dry period, which occurred at the end of the sixteenth
century. And the deep drought during the mid-nineteenth century that
peaked in 1860 was followed by the catastrophic 1861-62 l ood. More recently,
the drought of 1987-92 was followed by the unusually wet winter in 1993,
leading to massive mudslides, landslides, and record l ooding in some parts
of the West.
