EISEVIER
Geomorphology
19 (1997) 333-347
Response of eolian geomorphic systems to minor climate change:
ex.amples from the southern Californian deserts
Nicholas Lancaster
*
Desert Research Institute, UCCSN, P.O. Box 60220, Reno, NV, USA
Revised 6 May 1996; accepted 28 August 1996
Abstract
Eolian processes alnd landfonns are sensitive to changes in atmospheric parameters and surface conditions that affect
sediment supply and mobility. The response of eolian geomorphic systems to minor climate change can be examined through
process-response models based on a combination of relations between short-term changes in climatic variables and eolian
activity and the geologic and geomorphic record of Holocene eolian activity.
At both time scales, eolian activity in southern Californian deserts is strongly controlled by variations in precipitation.
Wind energy is not a limiting factor in this region. Formation of eolian deposits is a product of climatic changes that increase
sediment supply from fluvial and lacustrine sources and may, therefore, be closely tied to periods of channel cutting and
geomorphic instability. During intervening periods, eolian deposits migrate away from sediment source areas and are
reworked, modified, and degraded. Remobilization of existing dormant dunes is a product of reduced vegetation cover and
soil moisture in periods of drier climates. The major control on these processes is decadal to annual changes in rainfall that
determine vegetation (cover and soil moisture content.
Keywords:
wind transport;
eolian features; California;
climate
1. Introduction
Eolian processes involve the mobilization,
transport, and deposition of material by the wind. They
operate within geomorphic
systems that form an
interrelated set of processes and landforms in which
sediment is transported by the wind from source
areas to depositional
sinks via transport pathways.
Two major components of eolian sediment transport
can be identified: (I) material of sand size (> 0.050
pm); and (2) silt- and clay-sized particles (< 0.050
km) or dust. Major differences occur in the pro-
* Fax: + 1 702 674 71557. E-mail: [email protected]
cesses by which these two types of eoliau sediment
are transported and deposited, although entrainment
of all particle sizes is governed by very similar
variables. Sources and sinks for sediment are linked
by a cascade of energy and materials which can be
viewed in terms of sediment inputs and outputs,
transfers and storages. Study of eolian processes
within a systems framework permits the assessment
of geomorphic responses to climate change on differing spatial and temporal scales, as well as focusing
attention on the internal and external links between
system components.
Because eolian processes and landforms are the
result of interactions between the wind and the land
0169-555X/97/$17.00
0 1997 Elsevier Science B.V. All rights reserved.
PZI SO169-555X(97)00018-4
N. Lancaster/Geomorphology
334
19 (1997) 333-347
cussion of the approaches to the long-term dynamics
of eolian systems that seem promising, followed by
two case studies from the deserts of southern California that illustrate different approaches.
2. Approaches
There are two main approaches to understanding
the response of eolian geomorphic systems to minor
climate change. First, the response to short-term
climatic variations (e.g., El Niilo) can be examined
by developing process-response
models based on
correlations between climatic variables (e.g., rainfall)
and eolian activity. Second, the geologic and geomorphic record of Holocene eolian activity can be
correlated with other forms of proxy paleoclimate
data to provide a model for responses on time scales
of millennia to centuries.
I
I
I
Fig. 1. Schematic view of the factors that may influence
response of eolian geomorphic systems to climatic change.
2.1. Relations between minor climatic changes and
sand and dust mobilization
I
the
surface, they are sensitive to changes in atmospheric
parameters and surface conditions that affect sediment supply and mobility (Fig. 1). Thus, wind action
is influenced by changes in wind strength and direction that may be the direct result of global or regional climatic changes as well as by changes in
vegetation cover and sediment availability that are an
indirect effect of climatic change.
In this paper, minor climatic change is defined as
short-term (decades to centuries) change that is on a
scale from the Neoglacial and Little Ice Age periods
at the most extreme to the limited duration of El
Niiio-Southern
Oscillation periods, the Sahel drought
of the 197Os, and the Dust Bowl of the 1930s at the
other extreme. In this context, the changes involved
are minor compared
to those associated
with
glacial-interglacial
cycles, yet are very significant in
regional terms.
Compared to the other fields of geomorphology
covered in this volume, eolian geomorphic systems
have been rarely studied in the detail that they
deserve, and many of the fundamental dynamics on
time scales of years to centuries are poorly known.
Consequently,
this paper begins with a general dis-
Rates of sand and dust emissions are controlled in
part by short-term and seasonal changes in wind
o!
0
.
10
20
30
40
50
60
70
Number of dust storm days per year
80
90
80
90
180
160
20
3
40
50
60
70
Number
ofdust storm days per yew
Fig. 2. Relations between dust storm
variables in Mauritania (after Middleton,
frequency
1989).
and climatic
N. L.uncaster/Geomorphology
strength and variables such as surface roughness,
moisture content, and vegetation cover that affect the
threshold velocity for sediment mobilization (e.g.,
Nickling, 1989). In time, feedback mechanisms between the surface and the wind result in the removal
of erodible particles and the development of a surface lag or armor of non-erodible particles. These
processes tend to make many surfaces self-stabilizing, except where repeated disturbance or re-supply
of sediment occurs (Nickling and Gillies, 1989).
Periods of geomorphic instability resulting from minor climatic changes may, therefore, affect eolian
processes through increased sediment availability as
well as changes in vegetation cover and soil moisture.
On a time scale of seasons to years, considerable
debate has been generated on the relations between
dust emissions and antecedent precipitation. Al-
335
19 (1997) 333-347
though intuitively expected, cause and effect relations have proved difficult to establish quantitatively.
A complex relation apparently exists between dust
emissions and controlling factors such as vegetation
cover, surface crusting, and disturbance. In Mauritania and Niger, the annual frequency of dust storms is
weakly correlated with rainfall from the previous
year (r2 = 0.28), but is more strongly correlated
(r2 = 0.56) with the average annual rainfall in the
previous three years (Fig. 2) (Middleton, 1989). Similar observations were made by Chepil and Woodruff
(1963) for the Great Plains. In the southwestern
U.S.A., Braze1 (1989) found poor correlations between annual and seasonal precipitation, the Palmer
Drought Severity Index, and frequency of dust events
at sites in Arizona and southeastern California (Fig.
3). The clearest signal to emerge was a relation
between winter moisture and subsequent annual dust
1
I
W Arizona
0 Calibmh
A Nevada
Mean annual precipitation (mm)
0
50
100
150
200
250
300
350
400
500
Mean annual precipitation (mm)
Fig. 3. Relations between dust storm frequency
and climatic variables in the American Southwest (after Brazel, 1989).
336
N. Lancaster/Geomorphology
frequencies (Braze1 and Nickling, 1986). Likewise,
MacKinnon et al. (1990) found a statistically significant correlation ( r = - 0.60) between winter precipitation and dust emissions in the following year at
Yuma, AZ. They noted a strong reduction in dust
emissions in the period 1983-1984 following heavy
rainfall resulting from the El Nice event of fall and
winter 1982. Relations between the frequency of
specific types of dust-generating
weather and dust
emissions have also proved difficult to establish,
partly because of the variability
in wind speeds
between individual events of the same synoptic type
(Braze1 and Nickling, 1986).
2.2. Controls of dune mobility
Sand dune activity (expressed as changes in migration rates and/or variations in the amount of sand
movement on the dune itself (Thomas, 1992) is
directly proportional to the sand-moving
power of
the wind, but inversely proportional to the vegetation
cover (Ash and Wasson, 1983) and soil moisture
content. In turn, vegetation cover and soil moisture
are a function of the ratio between annual rainfall
(P> and evapotranspiration
(El. An index of the
sand-moving power of the wind is the percentage of
the time the wind is blowing above the threshold
velocity for sand transport ( _ 5 m/s) (W ). The ratio
between these two terms gives a mobility index:
(Lancaster, 1988):
W
M=-.--
P/PE
In this index, actual evapotranspiration
is replaced by
potential evapotranspiration
because this term can be
estimated from temperature data. Critical values for
M, based on field observations of dunes in southern
Africa, indicate that dunes are fully active when M
exceeds 200; lower slopes and interdune areas are
stabilized by vegetation when M lies between 100
and 200; only dune crests are active with M between 100 and 50; and dunes are completely stabilized when M is less than 50. Good relations between modem conditions of dune activity and these
climatic parameters have been established in the
Sahel region of Africa (Talbot, 19841, in Australia
(Ash and Wasson, 19831, the Kalahari (Lancaster,
19 (1997) 333-347
19881, and the Great Plains of the United States
(Muhs and Maat, 1993).
The effects of changes in the intensity and/or
frequency of eolian transport events on dune mobility are not well known. In Australia (Bowler and
Wasson, 1984) and the Kalahari (Lancaster, 1988)
studies indicate that many vegetation-stabilized
dunes
would be mobile if wind speeds were increased by
30%. In dune fields in the Sonoran Desert, and
possibly many other subtropical deserts, dune mobility is apparently reduced by present-day low wind
speeds.
Short-term changes in dune mobility have been
noted from the Gran Desierto, Sonora, Mexico and
the Algodones Dunes. A significant increase in the
area of bare sand and the area of small (2-3 m high)
crescentic dunes occurred in the northern Gran Desierto between 1988 and 1990, following a period of
low rainfall in this area. Smith (1980) described
year-to-year changes in the size of barchan dunes at
the Algodones dune field that can be related to
variations in the frequency of sand-moving winds as
well as the growth of annual or ephemeral plants in
interdune areas following rains. A combination
of
low wind velocities and growth of vegetation in
1978-1979
caused dunes to shrink as sand was
trapped in interdune areas.
Observations by early explorers may also provide
important information on the past extent of mobile
dunes. For example, such accounts indicate that parts
of currently vegetation-stabilized
dune fields from
Nebraska to Texas were active during the 19th century during periods of drought accompanied by higher
temperatures (Muhs and Holliday, 1995).
2.3. The Holocene record of eolian activity
Evidence indicating that eolian activity has been
both more extensive and/or
intense than it is at
present is widely distributed in currently arid and
semi-arid areas (Lancaster, 1990). Types of evidence
include dormant and relict dunes and sand sheets that
are stabilized by vegetation, soil development, colluvial mantles, and deflation lags (Muhs, 1985; Holliday, 1989; Tchakerian,
1991; Thomas and Shaw,
1991); ventifacts covered by desert varnish (Laity,
1992), and dust mantles in desert margin areas (Tsoar
N. Lancaster/Geomorphology
and Pye, 1987). Many of these landforms and deposits are the products of major climatic changes in
the Late Pleistocene and early Holocene.
Although Thomas and Shaw (1991) have suggested that many partially vegetated dunes may not
be relicts of past climates, an increasing body of
evidence demonstm1es the dynamic nature of many
dune systems which display multiple periods of stabilization and reactivation in the late Holocene. For
example, the cores of large complex linear dunes in
Mauritania consist of sand deposited during the period 20,000-13,000 yr B.P. and stabilized by vegetation and soil formation during a period of increased
rainfall 1l,OOO-4500 yr B.P., further periods of dune
formation after 4000 yr B.P. cannibalized existing
eolian deposits on the upwind margin of the sand sea
and the currently active ‘cap’ of crescentic dunes
superimposed on the linear dunes dates to last 30
years (Kocurek et al., 1991). Similar sequences of
dune deposits have been recognized in the Sahel
(Talbot, 1985) and1 southern Sahara (Viikel and
Grunert, 1990). Significant Holocene dune activity
has also been recognized in Australia (Nanson et al.,
1992).
In North Americ,s, currently vegetation-stabilized
dunes on the Great Plains were active after 9000 yr
B.P. with maximum eolian erosion and deposition
6000-4500 yr B.P. (Holliday, 1989). Similar
chronologies are recognized in Wyoming (Gaylord,
1990) with at least four episodes of enhanced eolian
activity after 7500 years ago: maximum aridity occurred 7545-7035 B.P. with a lesser peak 5940-4540
B.P. The Nebraska. Sand Hills have experienced
several periods of reactivation and/or dune formation in the Holocene (Ahlbrandt et al., 1983). In
many areas of the Great Plains, the most recent dune
activity was thought to have occurred between
3500-1500 B.P. (see summary in Muhs and Maat,
1993), but evidence for eolian activity in the past
1000 years is now becoming available (e.g., Madole,
1994). Periods of Holocene dune activity probably
occurred during warmer and drier episodes, with
prolonged drought between 6500 and 4500 yr B.P..
Periods of dune activity in the southwestern United
States are less well constrained, but Stokes and
Breed (1993) recognize three periods of dune reactivation 400,2000-3000 and 4700 years ago in northeastern Arizona.
19 (1997) 333-347
331
A major problem encountered in studies of
Holocene dune stratigraphy and chronology is the
difficulty of dating episodes of dune formation and
eolian deposition. Reliable dating of periods of dune
activity is critical, as many dune fields indicate
multiple episodes of dune formation. In some areas,
14C dating of organic materials in paleosols can be
used to bracket periods of eolian deposition (e.g.,
Forman and Maat, 1990; Wells et al., 1990; Kocurek
et al., 19911, but many eolian depositional settings
do not favor soil formation and/or preservation, or
organic matter content in these soils is too low.
Luminescence dating provides a means to establish a chronology of periods of eolian activity and
dune formation by measuring the time since burial of
sediments and/or stabilization of areas of dunes
(Wintle, 1993). The technique has been applied successfully to develop chronologies of dune formation
in Australia (Gardner et al., 1987; Lees et al., 1990)
and North America (Forman and Maat, 1990; Singhvi
et al., 1982; Edwards, 1993; Stokes and Gaylord,
1993). New developments of the technique that include stimulation of luminescence by photons in
infrared (IRSL) and green wavelengths (OSL) appear
to provide reliable and precise age estimates for the
deposition of eolian sands (Wintle, 1993). The availability of numerical ages for periods of eolian deposition also facilitates correlations with paleoclimatic
data from other sources, and leads to more appropriate determinations of the response of landforms to
climate change.
3. Case studies
3.1. Modem
the Coachella
climate variations and dune activity in
Valley, south-central California
Studies of the response of dunes to climatic variability on the scale of decades are rare. In part this is
because records of climate in arid regions are often
short, and long-term monitoring of dunes is rarely
undertaken. A recent study of dunes in the Coachella
Valley of southern California (Lancaster et al., 1993)
shows how data on changes in dune morphology and
migration rates derived from aerial photographs taken
over a fifty-year time span can be correlated with
information on rainfall and winds to derive a new
338
N. Lancaster/Geomorphology
understanding of eolian dynamics on a decadal time
scale.
The Coachella Valley of southern California contains some of the most active eolian terrains in North
America. Sand derived from washes and distal alluvial fans is transported southeast by strong westerly
and northwesterly
winds that are funneled through
the San Gorgonio Pass. Expansion of the surface
flow downwind of this constriction results in a rapid
decrease of wind energy toward Indio (Sharp and
Saunders, 1978) and deposition of sediments in the
area east of Palm Springs and the Indio Hills (Fig. 4)
where several areas of crescentic, parabolic, and
shrub-coppice
dunes occur (Beheiry, 1967).
The Indio Hills dune field (Fig. 5) consists of
well-defined
parabolic and crescentic dune complexes that comprise three linear, NW-SE-trending
19 (1997) 333-347
areas. These dune complexes are surrounded and
separated by regions of undulating sand sheets and
blowouts. The upwind (northwest) terminus of significant eolian sand accumulation is characterized by
degraded parabolic dunes and sand sheets. South and
east of the main dune areas, extensive, continuous
sand sheets characterized
by mesquite-anchored,
shrub-coppice
dunes can be found on the early
photographs. This area is now occupied by housing
developments.
Comparison of aerial photographs taken in 1939,
1953,1972, and 1992 reveals that significant changes
in eolian morphology have occurred within the Indio
Hills dune field. These changes include alterations in
eolian deposit form, location, and size (Fig. 6). For
example, the areas characterized by dune ridges increased from 1939 to 1953, and generally declined
Fig. 4. Location map for the India Hills dune field and the Coachella Valley, southern California.
N. Lancaster/
Geomorphology
thereafter. Both the western and eastern dune areas
have decreased in length over time (especially since
1953), but the width has changed only slightly,
19 (1997) 333-347
resulting in increases in the width-length (W/L)
ratios. The area characterized by sand sheets has also
declined with the most significant decrease occurring
Dune Ridges
Sand Sheets
r--.-------.‘~~meters
??
339
Vegetated Dune Areas
Agricultural Fields
Mesquite Groves
Fig. 5. The Indio Hills dune field, showing location of main dune complexes. Map shows dune field as it was in 1953.
340
N. Lancaster/Geomorphology
after 1972. This reflects thinning and disintegration
of the sand sheets into isolated regions along the
upwind areas of the dune field.
Fig. 7 summarizes changes in dune complex migration rates over time. The margin of the western
dune area advanced at an average rate of 27 m/yr
between 1939 and 1953. Between 1972 and 1992 the
western dune complex encountered wind breaks along
the margin of adjacent agricultural areas and the
migration rates slowed to only 13 m/yr. The eastern
dune complex migrated to the southeast at rates that
varied from 26 m/yr during the period from 1939 to
1953 to 39 m/yr between 1953 and 1972. Its migration was also hindered by wind breaks after 1982,
the rate of advance decreasing to 31 m/yr. A general decrease in dune migration rates from the period
of 1953-1972 to 1972-1992 is evident from the
response of the far-eastern dune complex (Fig. 7).
A
--s---West Dunes
-EastDunes
A- -FarEastDunes
. .+-
i.O-
0.6 -
..
+.’
. +- . _ .
..
! o’6_
4
+ ...... ~
0.4
0.2-
A-
t’--
-_
o.o1930
1.8
1.6
1.4
1.2
5
IO
0.8
1950
1960
1970
1980
1990
2000
B
4 +...
1
3
z.
1940
Y
-o-Active
dunes
.-+--3andsheels
...
'+.,
..
..
‘+
I/.'\-
0.6
0.4 1
0.2 ]
IQ30
Fig.
+
I
IQ40
1
1950
I
1960
I
1970
I
1980
I
IQ90
1
2000
6. (A) Changes in dune complex area, and (B) areas of active
dunes and sand sheets in the Indio Hills dune field 1939 to 1992.
19 (1997) 333-347
+-W&dune
Eastdune
A- - FarEastdune
-
+-
45.0
t
40.0
.“\
1
g
35.0-
1
t?
30.0-
\‘ . .
\ . ..+
\
3
i
e
.z
\
\
\
20.0
\
15.0
25'o!
10.0
]
Fig. 7. Changes
1939 to 1992.
'\--;
L
I
I
1939-1953
1953-1972
I
1972-1992
in Indio Hills dune migration
rates over time,
Unlike the other two dune complexes, however, the
far-eastern dune area is unconstrained by wind
breaks.
A 20 year record of wind data collected at the
Palm Springs airport, reveals that the typical, seasonal patterns of sand transport observed in other
parts of California (e.g., Laity, 1987) apply to
Coachella Valley. For example, winds capable of
moving sand (> 5 m/s) are mainly from two directional sectors: west to northwest (38.15% of all
winds) and east and east-southeast (17.69% of all
winds). The single most important sand-moving wind
direction is west-northwest (23.19% of all winds)
and is associated with the late spring to early summer period. The easterly sector is most important
(25-29% of the winds) in July through September,
reflecting the influence of monsoon air movement
from the Gulf of California. Mean wind speed over
the year is 5.09 m/s, with the highest values (up to
9.1 m/s hourly average at a height of 10 m) occurring from March to June. The data for dune migration presented above suggest that rates of eolian
activity and sediment transport have varied over
periods of years to decades. These relations can be
assessed using the dune mobility index (Lancaster,
1988). Dune mobility indices for the period 19731991 were calculated using climatic data collected at
Palm Springs (Fig. 8). Unfortunately, wind data predating 1973 were not available for Palm Springs,
prohibiting the calculation of the dune mobility index for the earlier part of the period covered by the
aerial photographs.
N. Lancaster/Geomorphology
A
50
1
30,~,
, ; , , , , , , , , , , , , ,
197419701978198019821984198819881990
B
P
250
3
200
150
100
50
0
rrrrrrrrrrr--r-r-.-.-
C
2500
cient of variability for precipitation was 66.69%.
This suggests that the variability in precipitation is
the main factor affecting changes in the mobility
index from year to year. Fig. 8 also suggests that
periods of higher rainfall tend to be less windy than
relatively dry spells. It is also possible to assess the
importance of total annual precipitation on sand
transport by comparing totals of yearly rainfall with
the rates of dune complex migration for the periods
of 1939-1953, 1953-1972, and 1972-1992. Except
for the west dune, the comparisons (Fig. 9) show that
the wet intervals of the late 1930s and 1940s as well
as those following 1976 generally exhibit relatively
lower rates of dune complex migration.
The question of why wind strength does not have
more of an influence on the variability of eolian
sediment transport can be explained in terms of
limiting conditions. For the time scales considered
here (one-year intervals>, wind velocities capable of
sediment transport occur frequently enough every
year (an average of 36.85% of the time during the
period 1973-1992) to transport the available sediment and, therefore, the movement of eolian sand is
not limited by wind duration and velocity, but by
other parameters such as sediment moisture content,
sediment availability, and vegetation cover. Sediment moisture content and vegetation cover may be
directly controlled by annual precipitation, whereas
sediment availability may be determined by longerperiod variations in precipitation and stream channel
dynamics.
The probable source areas of sediment for the
Indio Hills dune field are the modem washes on the
-u-
o+ll-rrt
I
t
t
t
t
I
I
t
I
t
t
t
t
tl
A sensitivity analysis was also performed to determine the primary factor(s) that influenced sand
transport during this 18 year period. Comparison of
the variability in climatic parameters that may influence the mobility index shows that the percentage of
the time the wind was above threshold at Palm
Springs varied by 12.70% and potential evapotranspiration varied by only 4.61%, whereas the coeffi-
Awagemigmtiinrate
.+.
-Precipitatiin
35.07
197419761978198019821984198619881990
Fig. 8. Variations in percentage of time: winds blowing in excess
of 5 m/s, annual precipitation, and the dune mobility index for
Palm Springs, CA, 1973 to 1991.
341
19 (1997) 333-347
H
r*O
-13
30.0-
15.0’10
1939-1953
1953-1972
1972-1992
Fig. 9. Relations between average annual migration rate for the
Indio Hills dune field and mean annual precipitation (at Palm
Springs).
342
N. Lancaster/Geomorphology
alluvial fans derived from the nearby Indio Hills.
These washes feed sand streaks that extend to the
dune field. It appears, however, that sediment supply
has been depleted over the period since 1939: the
area covered by dunes and sand sheets has declined
and the trailing margins of the dune complexes are
moving away from the alluvial fan areas and disintegrate. Some of the largest flood events in the last
century have occurred during the time since 1939,
yet significant episodes of dune formation have not
taken place. One plausible hypothesis is that phases
of sediment input to dune fields and dune building in
this area (and possibly in many other parts of the
southwestern United States) are related to episodes
of valley floor dissection. Periods of valley aggradation and entrenchment are well-documented
throughout the region (for a review see Miller and Kochel,
1997). Periods of entrenchment appear to be associated with relatively minor increases in total annual
precipitation
and the frequency of intense storms
(e.g., Hereford, 1984; Balling and Wells, 1990).
Dissection of valley fill in response to these changes
provides sediment to distal alluvial fan areas, where
it can be redistributed by the wind to dune areas. As
19 (1997) 333-347
channels in the valleys reach some form of equilibrium, the quantity of sediment delivered to the alluvial fans decreases and the eolian geomorphic system changes from a period dominated by dune accumulation to one dominated by dune migration, modification, and degradation. Because precipitation is a
predominant
control on dune mobility, periods of
more rapid dune migration and modification will be
associated with drier intervals.
3.2. Holocene
jaue Desert
eolian activity in the east-central
Mo-
The area between Afton Canyon and the Providence Mountains in the eastern Mojave Desert (Figs.
10 and 11) provides an excellent example of an
eolian sediment transport system that has been affected by direct and indirect effects of climatic
changes on a variety of temporal scales. The fan-delta of the Mojave River as it exits Afton Canyon is
the principal source of sediment for sand that has
been transported southeastwards for u 50 km to the
depositional sink of Kelso Dunes (Sharp, 1966). The
Devil’s Playground, an area of sand sheets, climbing
and falling dunes, and small crescentic dunes, forms
km
Fig. 10. Location map for the Kelso eolian eolian sediment transport system in the east-central
Mojave Desert.
N. Lancaster/Geomorphology
19 (1997) 333-347
343
Fig. 11. Landsat TM (band 5) image of the Kelso dunes eolian sediment transport system.
the corridor through which sand is transported from
the Mojave River sink to Kelso Dunes. Currently,
mobile crescentic dunes and active sand transport in
the region are restricted to the western parts of the
Devil’s Playground
(Smith, 1984) and the highest
parts of the Kelso Dunes (Paisley et al., 19911, and
no sand from the Mojave River reaches the main
dune mass. A well-developed
vegetation cover of
grasses (mainly galleta grass) and shrubs (principally
creosote bush) occurs on the inactive and relict sand
surfaces.
Evidence indicating that eolian activity has been
both more extensive and more intense than it is at
present occurs throughout the Kelso eolian sediment
transport system and includes: vegetation-stabilized
dune systems and sand sheets, relict climbing and
falling dunes (sand ramps), and eolian sand deposits
intercalated with alluvial fan sequences (McDonald
and McFadden, 1994). Other evidence includes eolian sand mantles on beach ridges at Silver Lake
(Wells and McFadden, 1987) and ventifacts covered
by desert varnish (Laity, 1992). Studies of stratigraphy and luminescence
dating indicate that a major
change occurred in the eolian depositional environment in the Mojave Desert during the early Holocene.
Prior to this time sediment supply from fluctuating
lakes was apparently sufficient to promote the accumulation of large climbing and falling dunes, even in
relatively mesic climates compared to today (Rendell
et al., 1994). After the desiccation of the region in
the latest Pleistocene and early Holocene, no further
sediment was available from lacustrine sources, because the available sand-sized material had been
deflated. Many of the sand ramps ceased to accumulate after around 8 ka, and were then stabilized by a
talus cover and later trenched by ephemeral streams.
344
N. Lancaster/Geomorphology
Holocene eolian activity was apparently restricted to
the major dune fields and immediately adjacent areas
(Edwards, 1993; Clarke, 1994) and to areas of active
sediment supply (e.g., adjacent to the Mojave River;
Laity, 1992; Clarke et al., 1996).
Kelso Dunes form the main depositional sink for
eolian sand in the region. The dune field covers an
area of approximately
100 km2 at an elevation of
500-900 m on the Piedmont alluvial deposits of the
Granite and Providence
Mountains
(Sharp, 1966;
Lancaster, 1993) and consists of a 40 km2 area of
active dunes surrounded on the west, north, and east
sides by areas of lower vegetation-stabilized
dunes.
Luminescence dating of eolian sediments in alluvial
fan sequences southeast of the main dune field suggests that the Kelso Dunes are at least 20,000 yr old
(Clarke, 1994). A major period of dune formation
also occurred 8400-10,400
yr ago. Luminescence
ages from areas of crescentic dunes on the northern,
eastern, and southwest margins of the dune field
(Edwards, 1993) indicate a later period of dune
formation prior to 4000 and again around 1500 yr
ago, with reworking of dunes between 800 and 400
yr ago (Wintle et al., 1994). Eolian sand also accumulated east of the main dune field in aggrading
alluvial fan deposits around 4250 yr ago (Clarke,
1994).
The Cronese Basins, located north of the Mojave
River Wash, are fed by flow in the ephemeral Mojave River as a response to increased rainfall in the
San Bernardino
Mountains
(Enzel et al., 1992).
Downstream
of Afton Canyon the Mojave River
19 (1997) 333-347
channel bifurcates, one channel flowing east to Soda
Lake, the other flowing northward along the base of
Cave Mountain into the East Cronese Basin. Seven
paleoshorelines
have been identified
from the
Cronese Basins, and pits and cores throughout the
East Cronese Basin show alternating lake and playa
conditions
with periodic
Mojave
River floods
(Brown, 1989). A narrow spillway connects the East
Cronese Basin to the West Cronese Basin with water
flowing between the two basins once the lake level
in the East Cronese Basin reaches a minimum elevation of 2 m (Brown, 1989). Eolian activity in the
Cronese Basin probably reflects deflation of sediment deposited by lakes fed by high floods in the
Mojave River.
Results of luminescence
dating from the West
Cronese Basin and the Devil’s Playground (Rendell
and Sheffer, 1996; Clarke et al., 1996) indicate that
dune formation has occurred episodically throughout
the Holocene (Fig. 12). Significant periods of eolian
deposition
occurred
approximately
6800-5500,
1400-2300, and 200 years ago. Material from a 1
m-high yardang at the south side of the West Cronese
Basin near the spillway, gave an age of 250 + 75
B.P., showing that large-scale erosion and transport
of sand from the West Cronese Basin has occurred
since that time (Clarke et al., 1996).
Early-mid-Holocene
eolian activity in the region
was probably a response to enhanced sediment supply from former paleolakes and geomorphic instability resulting from the desiccation of the region at this
time (Wells and McFadden, 19871, together with a
WEST CRONESE
BASIN
SALCH
OLD DAD
MOUNTAINS
KELSO
DUNES
Fig. 12. Timing of major periods of Holocene eolian activity in the Kelso region, compiled from luminescence
(19941, Clarke et al. (1996) and Rendell and Sheffer (1996).
dating data in Wintle et al.
N. Lancaster/Geomorphology
period of dry climates 6800-5060 yr ago (Spaulding,
1991). The stabilization of dunes at Kelso and elsewhere in the region after 4000 yr B.P. was probably
the result of cooler and wetter regional climates that
resulted in increased vegetation cover. The period
4000-3000 yr ago appears to have been cooler and
wetter in many parts of the southwestern U.S.A. with
lowering of the woodland-desert shrub boundary
(Spaulding, 1985; Spaulding et al., 1994), increased
groundwater recharge in southern Nevada, and shallow lakes in Death Valley, Searles Lake, and the
Silver Lake Basin (Enzel et al., 1992). The latter was
probably fed by increased winter rainfall in the San
Bernardino Mountains (Enzel et al., 1989).
Limited paleobotanical and dendroclimatological
evidence from southern California suggest that relatively dry conditions persisted at around 1450 yr
B.P. (Spaulding et al., 19941, and from 850 to 450 yr
B.P. (Fritts and Gordon, 1982). A radiocarbon-dated
beach ridge with an age of 390 f 90 yr B.P. indicates that a permanent lake again developed in the
Silver Lake basin at this time as a result of cooler
and wetter conditions and increased rainfall coeval
with the Little Ice Age in Europe (Enzel et al.,
19921, giving rise to’renewed stabilization of dunes
at Kelso. Eolian activity prior to 200 years ago in the
West Cronese Basin may reflect deflation of sediment deposited by shallow water bodies in this wetter period.
In summary, luminescence-dated evidence for
Holocene eolian actjvity in the east-central Mojave
suggests multiple periods of dune activation and
stabilization as climates in this region crossed and
recrossed mobility thresholds. Accumulation of new
areas of dunes appears to have been promoted by
increased sediment supply from fluvial and lacustrine sources.
4. Conclusions
Studies of dune dynamics in southern California
on two widely differing temporal scales provide the
basis for a tentative model for the response of eolian
geomorphic systems to minor climate changes. At
both the Holocene ;and decadal time scales, eolian
activity is strongly controlled by variations in precipitation. Wind energy is apparently not a limiting
19 (1997) 333-347
345
factor in this region as winds are above threshold
speeds for transport for as much as 30% of the time.
Formation of eolian deposits is a product of climatic
changes that increase sediment supply from fluvial
and locally lacustrine sources. Dune formation may,
therefore, be closely tied to periods of channel cutting and geomorphic instability and is therefore
highly episodic. During intervening periods, eolian
deposits migrate away from sediment sources and
are reworked, modified, and degraded. Remobilization of existing dormant dunes is a product of reduced vegetation cover and soil moisture in periods
of drier climates. The major control on these processes is decadal to annual changes in rainfall that
determine vegetation cover and soil moisture content. Vegetation cover is probably the single most
important determinant of eolian activity at this scale
and acts through direct protection of the surface and
absorption of momentum from the wind (Wolfe and
Nickling, 1993) to increase the threshold wind shear
velocity for transport.
Acknowledgements
Research on aspects of eolian dynamics in southem California has been supported by the Department
of Energy, National Geographic Society, National
Science Foundation (EAR 92046481, NATO Collaborative Research Grants Program, and the Nature
Conservancy. I thank my collaborators in these projects: especially A.G. Wintle and M.L. Clarke (Institute for Earth Studies, University of Wales,
Aberystwyth) and Helen Rendell (University of Sussex> for luminescence age determinations, A.J. Braze1
and A. Bach (Arizona State University) for climatic
data, and J.R. Miller and Lynn Zonge (Desert Research Institute) for assistance with aerial photography and climatic data analysis, interpretation, and
mapping.
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