Geoscience Reference
In-Depth Information
Sullivan and Byrne took a series of sediment cores from beneath Little
Packer Lake and brought them back to Byrne's laboratory. During the rou-
tine X-ray analyses, they noticed some unusual bright layers in the cores that
turned out to be rich in river-borne sand and silt. h ese denser sand layers
shone white next to the darker, more organic layers produced by algae grow-
ing in the lake during normal (non-l ood) years. Sullivan and Byrne reasoned
that, since the lake had no direct hydrologic connection to the river except
during major l oods, these deposits must have occurred when the Sacramento
River overtopped its banks and carried suspended sediments into Little
Packer Lake. Because larger l oods transport more sediment, the thickness
of the sand layers provides an approximate measure of the relative size of the
l ooding event. Sullivan and Byrne also compared the l ood layer thicknesses
to l oods of known magnitude, such as the catastrophic 1861-62 event.
Eager to understand more about how these l oods i t into California's
natural history, Byrne and Sullivan used radiocarbon analysis to date the
cores and estimate the ages of the l ood layers. Although these ages lack the
precision of ages that are determined by counting annual varves in the Santa
Barbara Basin, they can still provide an approximate measure of the l ood
frequencies. h e researchers calculated that, over the past 800 years, l oods
at least the size of the 1861-62 event struck Northern California on average
every 100 to 120 years.
h is is bad news to state water planners, who have estimated that the 1861-
62 l ood was a “500- to 1,000-year event,” which was determined by plotting
known l ood magnitudes against their recurrence intervals from California's
Central Valley over the past century. h is so-called l ood frequency curve
then allows hydrologists to extrapolate the recurrence inter vals of much larger
l oods (like the 1861-62 event) using their estimated magnitudes. Because
paleol ood records provide actual data about the magnitudes and recur-
rence intervals, they should be considered along with those estimated from
the l ood frequency curves. For instance, the research of Byrne and Sullivan
revealed evidence of four even larger l oods that dwarfed the 1861-62 event
over the past 800 years. Like Schimmelmann with his results from the Santa
Barbara Basin sediments, Byrne and Sullivan found evidence of megal oods
recurring in the region approximately every 200 years.
Evidence of megal oods has also emerged from the far northwest corner
of California. In that mountainous region, l oods ravage the steep, narrow
canyons in the Klamath and northern Coast Ranges, where l ood debris tends
to form thicker, coarser layers than are found in the broader and more gently
