Will Rose

Geography 810

Spring 2005

Climate controls on aeolian activity and sediment supply in

desert environments with an example of the Kelso Dunes,

Mojave Desert, California

Abstract

Desert aeolian sand transport systems generally rely on fluvial systems

for their sediment supply. The coupling of fluvial and aeolian systems

can be a rather complex affair, with climate variability exerting

enormous influence over the coupled geomorphic system. Desert

environments are particularly sensitive to environmental changes, with

the fluvial and aeolian geomorphic systems and their resultant

landforms reflective of that. The Mojave River / Kelso Dunes sand

transport pathways in the eastern Mojave Desert of southern California

provide a clear example of geomorphic response to changes in climate

regimes over a wide range of timescales.

Introduction

Climate exerts substantial influence on aeolian geomorphic systems, though not

solely through the control of wind speed. Fluvial systems are influenced a great deal by

climate regimes as well, and the sediment exchange between the two types of geomorphic

systems in desert environments is where much of the connectivity between the two can be

observed. Many geologists and geomorphologists have documented the existence of

aeolian and fluvial deposits together in sedimentological records (Langford and Chan,

1989), but early geomorphology traditionally viewed aeolian and fluvial systems as

separate realms, operating exclusively of one another (Bullard and Livingstone, 2002).

Early dryland geomorphology was divided over which process was the dominant land-

forming one, pitting the ‘aeolianists’ against the ‘fluvialists’ (Bullard and McTainsh,

2003). In desert environments, however, fluvial and aeolian geomorphic systems are

intimately intertwined, and the relationships between the two types of systems are

inherently complex and controlled in large part by climate variability, operating on a

variety of temporal and spatial scales.

Recently, more papers have been focused on the interactions between aeolian and

fluvial systems (e.g. Lancaster, 1995b; Muhs, et al, 2003; Rendell, et al, 2003). In Great

Sand Dunes National Monument in Colorado and in the Mojave River Wash area in

California, Langford (1989) observed six major ways that aeolian and fluvial systems

interact: (1) streams were dammed by aeolian deposits; (2) interdune areas were flooded

by stream waters; (3) dunes immediately adjacent to flooded interdune areas and channels

were eroded; (4) fluvial sediment was deposited in interdune areas; (5) groundwater from

the fluvial system flooded interdune areas; and lastly (6) wind eroded fluvial sediment

transferring it into the aeolian system. The focus of this paper is the last, that sediment is

delivered to the aeolian system by fluvial processes, and the role climate variability plays

in this interaction.

The Nature of Aeolian Transport

As does water in fluvial systems, wind entrains sediment when velocities reach a

certain threshold, and deposit s sediment, forming dunes and other depositional

landforms, when velocities decrease past a certain threshold. Sands are most often

transported by saltation, and the threshold velocities for sand transport by wind have been

studied at great length (e.g., Bagnold, 1941). Controls on aeolian dune activity, in

addition to wind speed, include surface moisture, vegetation, and sediment supply

(Lancaster, 1994).

Lancaster (1997) developed a dune mobility index (M) based on these controls,

which can be written as follows:

M = W / (P/PE)

where W is percentage of time wind speeds are above a transport threshold, P is annual

precipitation, and PE is annual potential evapotranspiration. The drier the area, and the

higher the wind speeds, the more sand transport will occur, given the availability of

sediment.

Figure 1. Relationship between sand transport (u* is shear velocity) and moisture content (W). (From Lancaster, 1995a)

Climate variability obviously affects aeolian systems via the wind speed variable, but

other aspects of climate influence aeolian systems and their interactions with fluvial

systems. As can be seen in Figure 1, wet sand greatly impedes transport by increasing the

shear velocity (the threshold velocity required to entrain sand), though the role of

evaporation in this situation is still unclear (Lancaster, 1995a; Namikas and Sherman,

1995). Availability of water plays an instrumental role in Lancaster’s (1997) mobility

index, controlling sand movement by encouraging vegetation growth and wetting surface

sands. In the Coachella Valley, Lancaster (1997) found that precipitation was the most

important variable in affecting the mobility of the dunes, as did Lancaster and Helm

(2000) for other parts of the desert southwest of the U.S. (see Figure 2). In studies

precipitation’s relationship to sand transport, Lancaster and Helm (2000) found that the

annual precipitation maximum corresponded with the transport minimum. Moisture

availability is the most important control on aeolian sand transport (see Figures 1 and 2),

since dune-stabilizing vegetation and the cohesive properties of wet sand are both under

its influence (Bullard and Livingstone, 2002; Namikas and Sherman, 1995).

C

Figure 2 (previous page). Temporal variations in (A) annual precipitation, (B) percentage of time wind is above threshold (W), and (C) sand transport for areas in the southwestern United States (from Lancaster and Helm, 2000)

Precipitation’s effects are manifested in the shorter term in sand moisture content and in

the emergence of dune-stabilizing vegetation. However, this aspect of climate also exerts

substantial influence over sediment supply.

Sediment supply is not a part of Lancaster’s (1997) index, since for a dune to be

active, the wind can simply be reworking existing aeolian sand deposits. The aeolian

system’s sediment source can be either external or internal to the dunes. External sources

involve the deflation of an alluvial fan and transport of sediments along a pathway to the

depositional area, for example, and internally sourced sands often involve cannibalized

dunes or the deflation of interdune areas (Kocurek and Lancaster, 1999). Dune sands are

very rarely the result of primary work by the wind on rock, and instead are delivered to

the aeolian transport system via lacustrine, fluvial and alluvial processes (Kocurek and

Lancaster, 1999). Muhs, et al, (1996) provide an example of the role of major rivers in

supplying sediment to aeolian systems. Dune fields in northeastern Colorado, which were

previously thought to be made up of sand eroded locally from the Ogallala Formation,

were found instead to have formed from sediment from the South Platte River (Muhs, et

al, 1996). In their study of the historic dune activity on the Great Plains of the United

States, Muhs and Holliday (1995) found that the rivers of that region (including the

Platte, North and South Platte, and Arkansas) were previously braided, intermittent

systems, allowing their beds to be exposed to aeolian transport. The change in channel

form and flow regime, away from braided and toward single, deeper channels allowed

vegetation to stabilize exposed bed sediments, protecting them from wind erosion (Muhs

and Holliday, 1995). Sediment sources for the northern and western dunes of the Gran

Desierto Sand Sea have been found to be point bars and terrace deposits of the lower

Colorado River, which transports a bedload of 30-40% sand size particles (Lancaster,

1995b). It is possible that changes in the river system caused a shift from sand

accumulation in the northwestern portion of the sand sea to the south, resulting in new

dune formations. Lancaster (1995b) also theorized that the Colorado River’s periodic

shift and subsequent diversion into the Salton Sea to form the ancient Lake Cahuilla cut

off sand supply to Gran Desierto, while increasing sediment supply to Algodones Dunes

to the north (See Figure 3) (Lancaster, 1995b; Muhs, et al, 1995).

Figure 3. Location of ancient Lake Cahuilla (dotted line), Algodones Dunes, and sand roses for adjacent areas. (From Muhs, et al, 1995)

The uplift of Mesa Arenosa, which forced the Colorado delta’s avulsion to the west,

resulted in exposed abandoned floodplain sediments, the source for some of the oldest

sand in Gran Desierto (Lancaster, 1995b).

Ephemeral streams of desert environments tend to experience high intensity, low

frequency flood events of a short duration, tend to have high rates of bedload transport of

sands, tend to lose water to the sandy substrate in the downstream direction, and thus tend

toward the formation of unconfined braided depositional features (Mountney, 2004).

These channel forms allow fluvial sediment to be exposed to and transported by the wind.

The Mojave River in southern California possesses many of these characteristics,

allowing for its close relationship to aeolian depositional features in the region.

The Mojave River and Kelso Dunes

Physical Setting and Transport System

The Mojave River (Figure 4) flows from its headwaters in the San Bernadino

Mountains north and eastward into the desert, where it loses most of its flow to ground

infiltration. Only large magnitude (and low frequency) flows make it through Afton

Canyon to deposit sediment at the terminal fan at the eastern end of the canyon. The

largest and rarest of floods flow out through and past Afton Canyon to fill the Soda Lake

basin. The flow at the USGS gauging station in Afton Canyon is often nonexistent in

summer months and otherwise typifies the low frequency/high magnitude regimes of

desert streams (Figure 5) (USGS, 2005; Webb, et al, 2001). The deposition of sediment

by the Mojave River has varied since the mid Pleistocene from a delta in a large shallow

lake over the Soda and Silver playas, to a terminal fan at the exit of Afton Canyon in

more arid current conditions (Kocurek and Lancaster, 1999).

Figure 4. Mojave River Basin and Kelso Dune system (from Tchakerian and Lancaster, 2002)

The Kelso Dunes are connected to the Mojave River’s terminal alluvial fan by the

Devil’s Playground, which consists of sand sheets, crescentic and climbing dunes. The

Kelso Dunes cover an area of approximately 100 km2 and reach heights of up to 160 m.

They sit at the southeastern end of a basin ringed by the Bristol, Providence and Granite

Mountains, some 50 km southeast of the fan of the Mojave River where it exits Afton

Canyon, at the western end of the basin (Lancaster, 1993; Sharp, 1966).

Figure 5. Mojave River discharge, 1929-2004 (USGS, 2005)

The sands that form Kelso Dunes were driven there primarily from the terminal

fan of the Mojave River and the desiccated lake bed of Soda Lake by the prevailing

westerly winds. Though winds are mostly from the west, topography exerts substantial

control on airflow in the region, and winds tend to flow along valley axes (Clarke and

Rendell, 1998), thus creating the sand transport corridor between the Mojave River and

Kelso Dunes that is Devil’s Playground. The Providence Mountains act as a significant

barrier to the sand-moving winds, thus forming the depositional area of Kelso Dunes

(Zimbelman, et al, 1995). The Kelso Dunes are maintained a dune complex rather than a

sand ramp (rising up the sides of the Providence Mountains) by sheetwash and channel

flow activity on the mountain slopes (Lancaster, 1993). No sand from the Mojave River

reaches Kelso Dunes currently, and the only active areas of significant sand transport are

the western end of Devil’s Playground and the highest portions of Kelso Dunes

(Lancaster, 1997).

Role of Climate Variability

Kelso Dunes is made up of a series of dune depositional phases, stacked upon one

another (Lancaster, 1993). These phases of dune deposition were found by Lancaster

(1993) to be related to phases of abundant sediment supply from the terminus of the

Mojave River and the desiccation of the lakes formed by high flows of the Mojave. It was

found, through the use of luminescent dating technology and the study of the grain size

distribution, that there were 4 major periods of sediment input to Kelso Dunes (Lancaster

1993). The main pulse of aeolian deposition was correlated through dating techniques

with the desiccation of Lake Mojave (12 000 to 9 000 BP), which was a perennial lake

occupying the Soda Lake Basin (Fig 4) in the late Pleistocene. In addition, increased

runoff from hillslope destabilization by vegetation changes (from mesic to arid

communities) resulted in significant alluvial fan deposition in the region. Other pulses of

activity at Kelso dunes can be correlated with fluctuating lake levels in the terminal

basins of the Mojave River (Figure 6) (Clarke and Rendell, 1998; Lancaster, 1993;

Lancaster 1997).

In the case of the Kelso Dunes, the limiting factor in dune formation was

sediment supply, as opposed to precipitation, which Lancaster (1997) found to be the

case in the Coachella Valley. In response to changes in climate, the Mojave River flooded

periodically, leaving ephemeral lakes at its terminus (Enzel and Wells, 1997). These

lakes then dried up as a result of a trend toward an increasingly arid climate, thus

generating the sand supply for the Kelso Dunes. Lancaster (1997) suggests that dune

formation can be tied to geomorphic instability in the fluvial systems which supply the

sediment to the aeolian systems.

Figure 6. Variations in the level of Lake Mojave and other lakes in the Mojave River system along with luminescence dated periods of aeolian sedimentation in the area. Sample numbers refer to luminescent dating work. (from Tchakerian and Lancaster, 2002)

Clarke and Rendell (1998) found that aeolian transport and dune formation can be

linked not necessarily to periods of extreme aridity, but rather to past periods of fluvial

activity. They state that “94% of all sand deposition in the Mojave [Desert] can be linked

to known periods of lake stands or flood events in this region” (Clarke and Rendell, 1998,

p529). The traditional association of dunes with extreme aridity in the southwestern U.S.

was shown to be too simplistic. As can be seen in Figure 6, aeolian deposition at the

Kelso Dunes tends to be associated with periods of intermittent lake stands at the

terminus of the Mojave River. McDonald, et al, (2003) suggest that rather than a

wholesale humidifying of the climate, that increases in sediment delivery to alluvial fans

are the result of increases in extreme storm and flood events. Since desert fluvial systems

move the most material with high magnitude, low frequency events, an increase in the

frequency of those events would allow for a substantial increase in the amount of

sediment deposited on alluvial fans in the region.

Discussion.

As Lancaster (1997) points out, precipitation is, as in his mobility index, the most

important control in dune activity, but its role is not solely as a limiting factor. In addition

to hampering dune activity through encouraging vegetation and wetting sand,

precipitation augments the discharge of streams, which can enhance dune building

activity through increased erosion and enhancement of sediment supply. Fluvial systems

also increase the efficiency of size selective aeolian transport by performing the sediment

sorting work (Bullard and McTainsh, 2003). Water’s role in desert aeolian activity is a

complicated one, exerting both positive and negative controls on the transport process.

The coupling of the aeolian and fluvial systems can be associated with temporal

climatic change or spatial change, such as the spatial change between the San Bernadino

Mountains and the Mojave Desert (Bullard and McTainsh, 2003). The onset of dune

formation and activity has been related to fluvial activity in other currently arid regions of

the world as well. For example, Juyal, et al, (2003) used luminescence dating technology

to link aeolian deposition on the southern edge of the Thar Desert in India to a previous

fluvial phase. Rendell, et al, (2003) found that the timing of climbing dune formation in

Niger was episodic, with pulses of sand accumulation, similar to those examined in the

Kelso Dunes, linked to more humid times. Soil development occurred in the more arid

periods, when the dune was starved of sediment (Rendell, et al, 2003).

As can be seen from the case of Kelso Dunes and the Mojave River, as well as

other locations, the association of sand dune building episodes with extremely arid

conditions alone is an oversimplfication. Climate exerts influence on aeolian systems

both directly and indirectly through fluvial systems on a multitude of scales temporally

and spatially. Increases in precipitation can result in increased stream discharge and

increased sediment delivery to the alluvial fan, though an increase in precipitation trends

can also cause surface sands to remain wet enough to impede transport and encourage

vegetation to stabilize the dunes. Increases in precipitation near the headwaters of the

Mojave River, for example, due to orographic enhancement or climatic variability or

some combination thereof, could result in increased downcutting, geomorphic instability,

and thus more sediment deposited past the eastern end of Afton Canyon.

Fluvial and aeolian systems exchange sediment on the local scale as well,

especially where streams operate in proximity to dune fields, or dunes have formed at the

margins of stream channels (source-bordering dunes) (Bullard and McTainsh, 2003). At

Great Sand Dunes National Monument in Colorado, the aeolian sands mainly originate

from the Rio Grande River to the west. Medano Creek flows south along the eastern and

southern margins of the dunes, eroding sand along the way and transporting it to the

depositional lobe at its terminus, just upwind of the dunes. The local fluvial system

allows for the recycling and reworking of dune sands by eroding sand from the dunes and

depositing in the aeolian transport path. (Langford, 1989)

In addition to operating on and connecting different spatial scales, these systems

also respond on a variety of temporal scales. The climate variability and geomorphic

responses of the Mojave River and Kelso Dunes in the late Pleistocene and early

Holocene took place over millennia, but Lancaster (1997) found a relationship between

precipitation and dune migration over a period of 50 year for dunes in southern

California’s Coachella Valley. On an even shorter time scale, Muhs, et al, (1995)

observed “dramatic aeolian movement of sand” (51) from the washes of two rivers

draining the San Bernadino Mountains after the passage of a Pacific frontal system in the

spring of 1993.

A process-response model proposed by Muhs and Holliday (1995) illustrates the

complexity that is characteristic of these systems (Figure 7). Their model was developed

for the Great Plains region of the U.S., and it does not take into account some of what

was found in the Mojave region. For example, the Muhs and Holliday (1995) model

shows a decrease in precipitation leading to increased sand supply, which may not be the

case in the Mojave, since it has been found that precipitation is necessary for sediment to

be delivered to the aeolian system. This model, is a slight oversimplification of the

interactions of the variables. Increased aridity can, for a time, increase the availability of

the sand supply, but a shift back to a humid environment will be required to replenish the

sand supply that is exhausted under the arid cond itions. This model, like Lancaster’s

(1997) mobility index, is concerned with aeolian sand transport rather than dune-building

processes.

Figure 7. Process–response model of climate change and aeolian activity (from Muhs and Holliday, 1995)

Bullard and McTainsh (2003) present a conceptual model that better depicts the

role of climate variability in the functioning of aeolian systems (Figure 8). In their model,

we can see that it is the change from humid to arid that is most important for sediment

supply to a dune system. The humid phase allows the fluvial system to increase its

sediment delivery work, then the shift toward aridity makes that sediment available to the

aeolian transport system. As the climate becomes more arid, the sediment supply is

depleted and dune-building ceases, leading to erosion and re-working of aeolian deposits

by the winds.

Figure 8. Model of impact of climate phases on sediment production, availability and transport and the response of the aeolian dry system (from Bullard and McTainsh, 2003) Concluding Remarks

Climate’s influence on aeolian sand transport and dune formation is part of a

complex web of interactions between aeolian and fluvial systems, and the exchange of

sediment supply between them. To associate aeolian depositional landforms solely with

arid climate regimes is an oversimplification, since humid climates encourage the

generation of sediment to be delivered to the aeolian system. These relationships operate

on a wide range of temporal and spatial scales. Bullard and McTainsh (2003) point out

that, on a global scale, areas of aeolian activity can be closely associated with fluvial

systems. On a more regional scale, the Mojave River/Kelso Dunes transport system, for

instance, spans different climates along its course from the headwaters in the San

Bernadino Mountains through Afton Canyon and on to Kelso Dunes. The interaction

between Medano Creek and Great Sand Dunes in Colorado is an example of these

relationships on a local scale. It’s also been shown how the Mojave River/Kelso Dunes

system reacts to climate changes on long temporal scales, and Muhs, et al, (1995)

observed short time scale reactions of the aeolian and fluvial systems to the passage of a

storm front. The variety of scales on which these relationships play out adds to the

complexity of the systems and feedback mechanisms involved. Continued research into

these systems and their complexities is necessary given human kind’s alteration of the

earth’s climate. Investigation of the connections among geomorphic systems and climate

variability leads to important questions about the behavior of these geomorphic systems

and their responses to future climate scenarios and land-use changes.

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