Geoscience Reference
In-Depth Information
fans in those areas began well before any changes in plant cover upstream. They
also observed that sedimentation could occur during several quite different combin-
ations of vegetation change and concluded that factors such as local storm intensity
and changes in the routes taken by water and sediment on hill slopes, rather than
plant cover, probably controlled late Pleistocene and Holocene fan aggradation in this
region. Another study of alluvial fan activity in the Sonoran Desert, this time in the
Muggins Mountains near Yuma, showed that fan aggradation was widespread between
3.2 and 2.3 ka (Bacon et al.,
2010
). These authors concluded that such aggradation
was caused by rapid climate change and more intense El Ni no-Southern Oscillation
(ENSO) events (see
Chapter 23
for a review of ENSO). Curiously, they found no sign
of any historic reactivation of the alluvial fan surfaces despite rainfall records showing
many above-average precipitation events correlated with ENSO. Presumably the late
Holocene was a time of greater climatic extremes.
The examples adduced here show that it is hard to discern a clear climatic signal
from changes in river channel incision and aggradation, so each catchment needs to
be studied in its own right in order to tease out the influence of purely local factors on
stream behaviour.
20.7.4 Lake fluctuations
In the Great Basin, there are remnants of more than 100 former lakes, some of them
very large (Tchakerian,
1997
). The Great Salt Lake in Utah is the shrunken remnant of
Late Pleistocene Lake Bonneville, which covered more than 50,000 km
2
and was up
to 330 m deep (Gilbert,
1890
; Flint,
1971
). Lake Lahontan in north-west Nevada was
somewhat smaller (23,000 km
2
) and up to 275 m deep (Russell,
1885
), and Searle's
Lake in California is another of many smaller remnants of once large lakes (Flint,
1971
; Smith and Street-Perrott,
1983
; Lemons et al.,
1996
; Madsen et al.,
2001
).
By any standards, these were huge lakes. Gilbert (
1890
) observed the close spatial
association between high lake strandlines and glacial moraines and concluded that the
lakes were high during times of maximum glaciation. Other workers outside North
America reached similar conclusions during the 1860s with respect to the lakes of
the Near East and central Asia, such as the Dead Sea and other great water bodies in
central Asia, such as the Aral Sea, the Caspian Sea and Lake Balkhash (Flint,
1971
).
The notion that these 'pluvial' lakes were synchronous with glacials became very
firmly entrenched and persisted for nearly a century (see
Chapter 12
). The higher lake
levels were attributed to a combination of higher precipitation and reduced evaporation
from the lake surface caused by lower temperatures. Smith and Street-Perrott (
1983
)
collated the radiocarbon ages for high lake levels in North America and concluded that
most of the lakes inwhat are now arid and semi-arid regions were relatively high during
late glacial times. Attempts to resolve the relative importance of the different factors
controlling the lake water balance led to thorough hydrologic studies of individual
