G(OMDRPHOIUGYdust.ess.uci.edu/ppr/ppr_Gil96.pdf · 2. Eolian processes acting on playa systems The...

E L S E V I E R Geomorphology 17 (1996) 207-228

G(OMDRPHOIUGY

Eolian sediments generated by anthropogenic disturbance of playas: human impacts on the geomorphic system and

geomorphic impacts on the human system

Thomas E. Gill 1

Air Quality Group, Crocker Nuclear Laboratory and Department of Land, Air and Water Resources, University of California, Davis, CA 95616, USA

Received 25 May 1994; revised 26 February 1995; accepted 8 March 1995

Abstract

In many of the Earth's arid and semiarid lands, saline lakes, playas, and similar landforms are disturbed as a result of human activity. Diversion and/or consumptive use of surface or groundwaters has created the effect of a climate change in numerous drainage basins, resulting in the desiccation of lakes and reactivation of eolian processes at many locations. Playas are natural sites for extensive eolian activity because of the deposition of clastic and chemical sediments in basins by surface water (via fluvial transport) and groundwater (via efflorescence). Wind erosion and deposition of playa sediments has had a major role in the development of landforms and sedimentary units in the present (lunette fields worldwide; Simpson Desert, Australia) and geological past, from the Triassic (Mercia Mudstone, England) to the Quaternary (Lahontan Basin and Cima Volcanic Field, USA).

Anthropogenic distu~bance or desiccation of playa systems has resulted in the eolian transport of sand (e.g. Lop Nor, China; Konya Basin, Turkey; Rajasthan, India; Kappakoola, Australia; several sites in West Africa) and/or dust (e.g. Aral Sea, Kazakhstan/Uzbekistan; Old Wives Lake, Canada; Kara Bogaz Gol, (ex-)USSR; Lake Texcoco, Mexico; Owens (dry) Lake, Mono Lake and other playas, USA). Typically, this is accomplished by abstraction of water and/or removal of vegetation from terminal lake basins. An extensive review of the literature documents many examples and/or potential examples of such phenomena in numerous nations. The reactivation of eolian processes from closed basins produces air pollution in the form of fugitive dust (naturally occurring compounds released into the atmosphere by human actions), and has significant environmental and economic impacts on human activities in the surrounding areas. Restoration or mitigation of degraded land on or surrounding playas has been accomplished at Lake Texcoco, Kara Bogaz Gol and the Konya Basin, and is being actively implemented at Mono Lake, Owens (dry) Lake and the Aral Sea.

1. Introduct ion

Deserts and arid hinds are the primary regions of eolian activity on Earth, and the phenomenon and

1 Present address: Wind Erosion Research Unit, USDA-ARS, Route 3, Box 215, Lubbock, TX 79401, USA.

human impact of 'desert dust ' is well known (e.g. Prwr , 1981). At least 85% of natural desert surfaces, however, are no t susceptible to erosion by the wind (Wilshire, 1980). But when human activities modify the stability of such geomorphic surfaces, the area susceptible to wind erosion may be greatly expanded and the flux of eolian material suddenly increases.

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208 T.E. Gill / Geomorphology 17 (1996) 207-228

Reheis and Kihl (1995) state that cultivated land generally yields about 20% more dust than unculti- vated areas, and the dust flux in an arid region undergoing urbanization may be double the pre-disturbance level. Eolian sediments generated from anthropogenically disturbed lands are considered 'fugitive dusts' (naturally occurring materials released as aerosols as a result of human actions) subject to mitigation and regulation because of potential effects on visibility, ecosystems, and human health. Human impacts on terminal lakes, playas, and related landforms in arid or semiarid regions of Earth are changing the water budgets of drainage basins and disturbing geomorphic surfaces, thereby accelerating eolian processes which in turn have major environmental, economic and health impacts on the human system.

Permanent, ephemeral or desiccated bodies of water which have geomorphic systems subject to wind erosion range from tiny pans in Southern Africa and the southern Great Plains (USA) to huge bodies of water such as the Aral Sea. As treated in this paper, they include both terminal lakes and deposits in basins of internal drainage, and coastal sebkhas and related terrestrial landforms inundated ephemer- ally at present or in the recent geological past by water from the ocean.

No uniform nomenclature has been adopted by geomorphologists for these landforms. Bodies of water in arid regions, generally saline and in closed basins, and their dried sedimentary deposits are referred to by dozens of names throughout the world, according to language and culture: for example, 'playas' and 'playa lakes' in North America, 'salinas' and 'salars' in South America, 'chotts' or 'pans' in Africa, 'sabkhas' in the Middle East, 'boinkas' in Australia, and 'nor', 'kavir' or 'gol' in Asia. 'Sabkha' is the Arabic word for 'salt flat', whereas 'playa' is Spanish for 'beach'. The confusing terminology and the specific context of the many local terms is addressed in Rosen (1994). Geologists have used the terms '(continental) playa' or '(continental) sabkha' for the landforms described in this review. The word 'playa', as a general term for a "variety of topographic depressions and desiccated former lakes that occur in the arid zone" (Neal, 1975; Rosen, 1994) will be used herein, except when referring to specific sites with a different formal local name.

Many terminal lakes are remnants of significantly

larger pluvial lakes that existed during the Pleis- tocene. Eolian processes operating on them today can provide a link to geomorphic processes which took place when the antecedent lakes desiccated extensively as a result of global climate change at the end of the last glacial period. At present, human activities (mostly water extraction and diversion) are modifying the water budget of many terminal lakes, resulting in an 'artificial climate change' which des- iccates the lakes partially or completely into playas (Gill and Cahill, 1992a).

The detailed categorization of playas into characteristic types is beyond the scope of this paper, but has been dealt with by other investigators (Snyder, 1962; Neal, 1975; Motts, 1965; Mabbutt, 1977; Rosen, 1994). These studies have proposed classifications based on whether the basin is open or closed, whether surface water or groundwater dominates, whether erosion or deposition dominates, whether the groundwater is externally discharging, whether the playa surface is typically wet/dry, hard/soft, or smooth/rough, whether evaporites or clastic materials dominate the playa surface when dry, or even whether the playa surface can support a motor vehicle. The reader is referred to these works for a more complete discussion of playa classification; however, the most important factor controlling playa type is generally considered to be groundwater. Immature, active playas associated with desiccating or ephemeral saline lakes are generally characterized by a high level of geomorphic activity. If the water table is within several meters of the surface or if water under artesian head is discharging from the playa at depth, saline groundwaters may rise through sediments and discharge powdery, deflatable effiores- cences on the playa, forming 'puffy ground'; or, during periods of enhanced precipitation in its drainage basin, the playa can refill partially or completely with water.

2. Eolian processes acting on playa systems

The dynamic environment surrounding playa- terminal lake systems is highly conducive to the work of eolian processes. Playas are primary sinks for dust that settles on them or is washed into them by fluvial processes (Yaalon and Dan, 1974; Cooke

T.E. GiU / Geomorphology 17 (1996) 207-228 209

et al., 1993, p. 310). A playa or terminal lake that has its water supply :artificially maintained for stor- age purposes could be a net remover of eolian sediments. This paper, however, will deal with playas as a source of eolian sediments.

Coudr-Gaussen (1!984) described 'les cuvettes de sebkhas', dried salt lakes with disrupted, wind-erodible surfaces (Middleton et al., 1986), as an environment highly favorable for creating dust. Fine silts, which are easily deflatable and very capable of long-distance transpoJl as aerosols, are brought to the topographic low of the basin (the playa) by fluvial processes. Evaporite-cemented aggregates and cracked, curled edges of bound silt-clay 'skins' protruding into the air on the surface of playas are broken away in pieces by the wind and abraded into smaller particles. Salt weathering, which breaks down clasts into fines, is a major mechanism for the creation of wind-suspendible particles (Goudie et al., 1979; Pye, 1987, pp. 18- 21); sodium sulfate and sodium carbonate, two of the most effective salt weathering agents (Goudie, 1986), form industrially significant deposits in a number of dust-producing playas (i.e. Owens (dry) Lake, Lake Texcoco). Ice formation on playas during winter months can dis- rupt and destroy the surface, promoting wind erosion (Motts, 1970), especially during early spring when the ice melts and the surface first dries. The renewal of sediment sources during lacustral periods increases the flux of eolian sand and dust, enhancing other eolian processes such as ventifact creation (Laity, 1992, quoted in Lancaster, 1994).

Eolian entrainment of sediments from playas can be accomplished by either deflation or abrasion. In direct deflation, loose, fine particles on the surface, deposited by efflorescence of powdery salts or salt weathering of clastic sediments, may be suspended directly into the airstream. The abrasion mechanism requires coarser sediments moving in a saltation process; saltating sand grains cannot move long distances, but the impacts on the playa surface can disagglomerate fine ,flit- or clay-sized particles from the surface (Gillette, 1979) or break off fine chips from the saltating p~rticles themselves (Krinsley and Doornkamp, 1973). The resulting abraded fines are released into the air:~tream and may be transported long distances. Careful studies of the processes of dust generation on the Owens (dry) Lake playa

(Cahill et al., 1994, 1996) showed that the vast majority of airborne dust particles were generated by the saltation-abrasion process of salt-silt-clay crusts, and only a few percent at most were generated by direct suspension of loose fine sediments. Dune sands from the Erg du Djourab migrating across dry lake sediments of the Bodrl6 Depression in the Sahara (Servant and Servant, 1970) actively generate Harmattan dust via saltation, and the entrainment rates are increased by deflatable efflorescent salts atop the lake sediments (McTainsh and Walker, 1982; McTainsh, 1985). Williams and Zim- belman (1994) have even described the possibility of this process occurring at White Rock, an 'enigmatic' crater deposit on Mars with an apparently lacustrine past. Lakebed sediments entrained into the airstream by either mechanism may be transported and deposited downwind on local (within a drainage basin; Young and Evans, 1986) and regional (even continental) scales (McTainsh, 1989).

Human intervention, except when it may result in large-scale fluctuations in climate, is not necessary for the activation of eolian processes in most playa systems, large or small. Bell (1875), quoted in Last (1992), described the natural wind erosion of salts from small playas in the Northern Great Plains of Canada:

"During the dry season of autumn, the water evaporates completely from many of these ponds, leaving their beds covered by the dry white salts, which look like snow and are blown about in the wind."

Blackwelder (1931) was amongst the first to describe the operation of eolian processes accomplishing large-scale erosion of playas. He pointed out a number of important factors that make playas fa- vored sites for wind erosion, including: playas have flat, friable surfaces; "the wind is unhindered on the barren lake floor" (i.e. minimal roughness height); 'transient floods' redistribute material onto the playa surface, ensuring a continued supply of material available for eolian suspension; and the ability of sand in motion to abrade crusted clays, which curl into chips and cylinders as they dry out. All of these factors are now recognized as key geomorphic characteristics which make playas prime sites for eolian activity.

Many playas have been observed or referred to as

210 TE. Gill / Geomorphology 17 (1996) 207-228

producing plumes of dust from dry surfaces. Some examples in the Great Basin of the USA include: Alkali Lakes (Surprise Valley), California and Nevada; Buena Vista playa, Nevada (Wallace, 1961); Grass Valley playa, Nevada (Young and Evans, 1986); Deep Springs Lake, California (Jones, 1965); and Summer Lake, Oregon (Langbein, 1961). Garrett (1992) describes "relatively intense wind-blown alkali dust" from dry lakes in Wyoming and the Yukon Flats area of Alaska. In Chile, Stoertz and Erickson (1974) reported that salars "with extensive soft saline surfaces or soft granular gypseous crusts commonly trail elongate dust lobes of wind-blown material from the leeward margins." Katabatic winds deflate the Quaternary lakebeds of Afghanistan and eastern Iran, resulting in events characterized by very high velocity surface winds and dense dust palls (Middleton, 1986; Middleton et al., 1986). Lacus- trine sediments from the Lake Chad basin were identified by McTainsh (1985) as a major source of the Harmattan dust. Dulhunty (1977) described the large-scale deflation of dusts derived from fresh silts deposited during the intermittent fillings of the north arm of Lake Eyre, Australia, one of the largest playas on Earth. These silts and associated sandy sediments are blown into and form a major part of the sediments of the surrounding Simpson and Tirari Deserts. Dare-Edwards (1984) suggested that much of the loesslike sediments blanketing a large region of Australia were formed by such particles derived from desiccated playa surfaces in the geological past. The environmental geomorphology of lunettes (eolian-fluvial clay dunes downwind of desiccated lakes, formed of clay and clay/salt agglomerates derived from the playa surfaces) has been extensively documented and described by Bowler (1973) and Thomas et al. (1993).

On a larger scale, fine sediments exposed on the surface of desiccated remnants of large pluvial lakes in the Great Basin of the USA, such as Lake Lahon- tan and Lake Bonneville, were eroded by the wind as the climate became drier at the end of the Pleis- tocene. In some areas these fine sediments formed a significant component of the soil in the region downwind (Chadwick and Davis, 1980; Peterson, 1980). Morrison (1964, p. 102) described the complete desiccation of Lake Lahontan during the mid-Holocene, estimating that more than 2 km 3 of material was

removed from the Carson Sink by eolian processes, and Sack (1992) estimated that 11% of the lake sediments exposed to the surface in the Tule Valley subbasin of Lake Bonneville were removed by eolian processes. McFadden et al. (1984, 1986) investigated soils formed on well-dated lava flows in the Cima Volcanic Field of the Mojave Desert, and indicated that these soils were formed rapidly by short-term pulses of eolian dust produced by the desiccation of pluvial Lake Mojave in the Late Quaternary. Such processes most certainly have taken place throughout much of geologic time. For example, Talbot et al. (1994) suggest that a significant portion of the Late Triassic Mercia Mudstone Group of Somerset, Eng- land, including fine-grained clastics and evaporites, was formed by eolian sediments eroded from playas on a huge saline lake-alluvial plain complex that then covered much of northwest Europe. Smoot and Castens-Seidell (1994) interpret features of the Trias- sic lacustrine deposits of the Fundy basin (Canada) and Argana basin (Morocco) to represent effiorescent playa crusts with lenses of eolian deposits. Present-day eolian activity in terminal lake basins can provide an understanding of processes taking place in the geologic past. Bowler (1986) described the development of lunette-fringed lake basins in Australia as an analogue for Pleistocene processes, and Dorn (1990) discussed how alkalinity provided by dust from playas affects the trace-element geochemistry of rock varnishes and can be used as a local paleoenvironmental indicator.

3. Human impact on playa systems and eolian processes

Present-day human intervention with the water cycle can accelerate the desiccation of playas and the activation of eolian sediment transport processes which mimic those in the Pleistocene. Variations in albedo can lead to differential surface heating and a mesoscale thermal wind circulation around anthropogenically desiccated playas (Physick and Tapper, 1990; Tapper, 1991), having an effect on local eolian transport of sediments. Diminution of the water supply causes a lowered water table and/or lower piezometric surface, lowers the capillary zone, re- duces soil moisture and can reduce the vegetation

T.E. Gill/Geomorphology 17 (1996) 207-228 211

cover (Neal and Motts, 1967), making the playa surface more recepdw; to wind erosion. McDowell et al. (1991) documented the reactivation of lunettes in the northern part of the Great Basin of the USA, partly because of the withdrawal of water from individual basins for agricultural use. Saline dust plumes, referred to by local residents as 'white death', blow chronically off the Carson Sink in the Lahontan basin of Nevada, a playa extending over 700 km 2. This site is now extensively desiccated by water diversions (J. Smith and M. Polakoff, pers. commun., 1993; Mayer, 1993), potentially reminis- cent of the interpluvial eolian soil-forming pulses from pluvial Lake L~thontan (Chadwick and Davis, 1980). Cahill et al. (1994) describe the large-scale transport of dust from Owens (dry) Lake, California, desiccated by water diversions, and the deposition of dust at high altitudes in the White Mountains, more than 100 km downwind (Fig. 1). Marchand (1970) suggested that up to 34% of the total soil material at this locality was formed by allochthonous input of eolian materials, some of which is thought to have been transported by "'dust storms from the Owens Valley". The transport of fugitive dust from Owens (dry) Lake to the White Mountains today appears to

be accomplishing a similar process on a slightly smaller scale than dust transport from pluvial Lake Owens during the Pleistocene.

Much of the material carried by the wind in the desert consists of silt and sand eroded into the basins of terminal lakes. Although this material is generally unable to be transported far by the wind, when coarse sediments are remobilized they can have a primary effect by disturbing human settlements in the local area of past or present terminal lakes. These coarse materials often have an important secondary role in the creation of eolian fines, via the process of saltation which can abrade or disaggregate the surface into fine, suspendible material in the size range of approximately 0.1-30 ixm (Schroeder, 1985; Gomes et al., 1990). Fine clastic sediments, which are small enough to be potentially transported long distances by the wind, are most likely to be carried to and concentrated in the terminal lake in the center of the basin by fluvial transport. Those playas which have active near-surface saline groundwater systems may effloresce with powdery, fine crystals of evaporite minerals into 'puffy ground' that can be directly deflated by the wind.

At the present day, when terminal lakes undergo

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Fig. 1. Deposition of major elements as clay-size particles (< 2.5 I~m), spring 1994, at Schulman Grove, White Mountains, California, showing impact of major dust plume on 24 March 1993 (after Cahill et al., 1994).


Table 1 Examples of playas and related landforms where eolian sediments have been or may be (re)mobilized as a result of human activity

Nation Site a Cause Effects; Selected references

Turkey Great Konya Basin (A) overgrazing, overcultivation

Israel/Jordan Dead Sea (B) water diversion

Saudi Arabia Coastal sabkhas on overgrazing, devegetation Persian Gulf (C)

Algeria Chantrit depression (D) abandonment of agriculture

Tunisia Sebkha Kourzia (E) deforestation

Mauritania/ Senegal R. delta (F) overgrazing, overcultivation Senegal

Mali Lac Faguibine (G) water diversion India Sambhar Salt Lake and overgrazing, overcultivation,

others, Rajasthan (H) devegetation China Lop Nor (I) water diversion

(former) USSR Kara Bogaz Gol, water inflow restriction Caspian Sea (J)

Kazakhstan Lake Balkhash (K) water diversion

Kazakhstan/ Aral Sea (L) water diversion Uzbekistan Turkmenistan/ Sarykamysh (M) water diversion to Uzbekistan mitigate Aral Sea Australia Kappakoola (N) devegetation Mexico Lake Texcoco (O) water diversion

Mexico Laguna Salada (P) natural?

Canada Old Wives Lake, water overdraft Saskatchewan (Q)

U.S.A. Great Salt Lake, water inflow restriction Utah (R)

U.S.A. Great Basin playas groundwater and surface (at least 5, in at least water overdraft 4 states) (S)

U.S.A. Fort Rock Lake, devegetation Oregon (T)

U.S.A. Mojave Desert playas, abandonment of agriculture, California (U) off road vehicle abuse

U.S.A. Salton Sea, California (V) water diversion (accidental)/ water conservation

U.S.A. Mono Lake, California (W) water diversion

U.S.A. Owens (dry) Lake, water diversion California (X)

blowing sand; successfully mitigated (Cevik and Birkman, 1986) dust transport (Yaalon and Ginzbourg, 1966; Shatkay et al., 1988) blowing sand, dune formation (Anton, 1982) wind erosion, lunette formation (Boulaine, 1956) wind erosion, lunette formation (Perthuisot, 1989) remobilization of dunes (Tricart, 1954)

blowing sand (Tricart et al., 1960) blowing sand, saline dust storms; partially mitigated (Sharma, 1991) blowing sand and saline dust storms (Zhao Songqiao, 1986b) dust storms, mitigated (Shabad, 1984: Buinevich et al., 1993) potential dust storms? (Bond et al., 1992; Aladin and Plotnikov, 1993) severe dust storms (Micklin, 1988; Ellis, 1990) potential dust storms? (Kikichev et al., 1990) sand dune formation (Smith et al., 1975) severe dust storms; mitigated (Blanco, 1950; Jauregui, 1989) dust storms: mitigation plan considered (McClurg, 1994; Mueller and Rockwell, 1995) saline dust storms (Gomez et al., 1992)

fear of dust storms (Adams, 1964)

saline dust storms (Garrett, 1992)

wind erosion (Allison, 1966)

dust storms (Wilshire, 1980)

blowing dust and sand (Black, 1983; Schroeder et al., 1988) dust storms, partially mitigated (Kusko and Cahill, 1984; Cahill and Gill, 1987) severe dust storms (Ono and Cox, 1990; Gill and Cahill, 1992a,b)

a Letter in Fig. 2.


anthropogenic desiccation and form playas, the re- suiting geological process may constitute a violation of particulate air pollution regulations (Gill and Cahill, 1992a), and humans who chose to settle or grow crops in areas ,downwind of naturally or anthropogenically desiccating terminal lakes often find themselves fighting a losing battle with eolian sediments. Dust and sand generated by wind erosion can affect human health (Gomez et al., 1992), impede agriculture (Schroeder, 1985), affect sensitive ecosystems downwind (Cahill et al., 1994), imperil travel by motor vehicle and aircraft, and impede transmission of electrJ.cal power by high-voltage ca- bles (Bofah and Owusu, 1986). If global warming becomes a reality, decreases in runoff and increases in evaporation will hasten the process of desiccation of terminal lake basins (Goudie, 1993), increasing the potential for dust :storms.

4. Review of anthropogenieally desiccated playas giving rise to eolian ,erosion

A comprehensive review of the literature deter- mined examples of anthropogenically disturbed playas in which eolian processes have been reactivated. Examples, found in over a dozen nations, are

summarized in Table 1 and Fig. 2, and described by geographical region below.

4.1. The Middle East

In Turkey, Cevik and Birkman (1986) reported that excessive cultivation and overgrazing were among factors that led to the reactivation of eolian processes in the Great Konya terminal lake basin. Sand deposits were exposed and remobilized. In the surrounding dune fields, where natural vegetation was removed by overgrazing and wood collection, the sand was blown about the basin and resulted in a major environmental hazard. The desiccation of the basin "caused traffic accidents, made the living conditions unbearable to the people in Karapinar, and the population started to move to safer places away from the area".

Increased diversion of water from the Jordan River by Israel and Jordan, beginning in 1960, led to a steady decrease in the level of the Dead Sea until 1990 and exposed a band of desiccated sediments up to 2 km wide on the relict shorelands (Nishri and Nissenbaum, 1993). Wind erosion of these materials, while not extensive nor a major environmental hazard, does occur. Yaalon and Ginzbourg (1966) reported halite, believed to be derived from these 'salt

H

"~ o "~ . .~ r l . lr G

Fig. 2. World map showing locations of sites described in Table 1.

214 T.E. Gill/ Geomorphology 17 (1996) 207-228

flats', as a constituent of dusts falling on Jerusalem. Eolian entrainment of Dead Sea shoreland salts is significant enough that it must be considered in the calculation of the salt balance of the basin (Shatkay et al., 1988).

Coastal sabkhas--supratidal saline depressions formed by previously higher seas, and now occurring as extensive lagoons or bays near or barely above sea level--cover thousands of square kilometers along the shore of arid coasts such as the Red Sea and the Persian Gulf. Except for the coastal location, the geomorphology of the surfaces is essentially the same as that of playas discharging saline groundwater (Neal, 1975, p. 135; Pye, 1987, p. 69). Eolian processes are a major, active feature of these landforms (Schroeder, 1986; Warren, 1987, p. 49). On the eastern Saudi Arabian coast, overgrazing by cat- tle and the opening of water wells accelerated the removal of vegetation. As the eolian processes accelerated, a significant blowing sand problem and the formation of fields of barchanoid dunes developed in association with overgrazed areas along the sandy edges of the sabkha (Anton, 1982).

4.2. Africa

Several examples of fugitive dust from playas and related landforms have been reported from African nations. Boulaine (1956) published observations of the formation of a lunette, formed as a result of abandonment of salinized agricultural land in the Chantrit Depression, Algeria. Bowler (1973) reported of this study:

"The surface efflorescence of salts, over an area approximately half a kilometre wide, resulted in a breakdown of soil structure and development of a crust with a granular fabric rapidly eroded by high velocity winds. When observed by Boulaine in 1956, 50 years after the water-table change which initiated the soil erosion, the process had resulted in the growth of a clay ridge 1.5 m high."

In northern Tunisia, Perthuisot (1989) reported that deforestation of lands around Sebkha Kourzia (an inland playa) after the Arab conquest (approximately 1200 years B.P.) resulted in destabilization of geomorphic surfaces around the basin and deposition of a lunette. In the Senegal delta of Mauritania and

Senegal, Tricart (1954) reported that fossil red sand dunes in a basin of playas and ephemeral lakes were reactivated by overgrazing and overcultivation of peanuts on the dune surfaces. Tricart et al. (1960) noted that artificial regulation of the level of Lake Faguibine, Mali, was responsible for a small amount of eolian remobilization of dune sand.

4.3. Asia

Human-influenced desertification processes have resulted in wind erosion and deposition of sediments in saline lake basins in Rajasthan, India (Kar, 1986; Sharma, 1991). Sharma (1991) documented the destabilization of dunes and sand sheets as a result of human disturbance of the Sambhar Salt Lake basin, the largest saline depression in Rajasthan (Ghose et al., 1977). In the Mendha basin of the Sambhar drainage, windblown sand, activated partially by human intervention, has buried several tributaries draining into the lake and altered the surface drainage system. Removal of vegetation cover by excessive tree-cutting, overcultivation and overgrazing resulted in "the encroachment of sand masses upon the cultivated fields and infrastructure.., buildings were buried under sand encroachment", and other areas were rendered infertile because of windblown salt. Since 1980, a conservation and afforestation program has begun to restabilize the dunes.

In northwest China, remobilized windblown sand has covered tens of thousands of square kilometers (Smil, 1984); some of the material originated in basins covered by saline lakes in historic time and desiccated partially or completely as a result of human impacts on catchments (Williams, 1991), including Lop Nor (Wang Hangdao, 1987) and Taitima Lake in the Tarim River drainage (Zhao Songqiao, 1986a, pp. 70-76). Zhao Songqiao (1986b) described the complete desiccation of Lop Nor as a result of the diversion of water into the Tar im- Kongqi-Qargan canal:

"Misuse of water resources is another important cause of drifting sand hazard... A notorious example occurs in the lower reaches of the Tarim River and the Lop Nor area. Since 1972, the lower reaches of the Tarim R. and all its terminal lakes have entirely dried up, and the physical environment has conse-

T.E. Gill / Geomorphology 17 (1996) 207-228 215

quently greatly deteriorated, resulting in two 'too little' (water and vegetation), and two 'too much' (blowing sand and salt)."

In the early 1980s in the (then) Soviet Union (now Turkmenistan), 'salt storms' blew off the desiccated floor of Kara Bogaz Gol. Kara Bogaz Gol is a large, shallow bay connected by a narrow strait to the southeastern portion of the Caspian Sea, with one-way flow of water out of the sea into the bay. It is essentially a huge coastal sabkha, which in historical times had always contained some areas of dried saline lakes (Dzens-Litovskii, 1967: Neal, 1975, p. 135; Perthuisot, 1989). It desiccated completely into a sulfate-rich playa between 1980 and 1984 after the strait was deliberately cut off from the Caspian in order to facilitate industrial-scale harvesting of sulfate salts from the desiccated baylands (Shabad, 1984; Buinevich et al., 1993). Subsequently, the environmental threat (including that of dust storms) from Kara Bogaz Gol was noted. Partially recon- nected to the Caspian Sea by an aqueduct, the dust storm problem dimini shed.

Lake Balkhash, a shallow, narrow, large terminal lake in southeastern Kazakhstan, has been rapidly desiccating since the 1970 completion of a dam on the Ili River, its major tributary and diversion for irrigation of most o~Lher streams draining into the lake. Salinity of Lake Balkhash has rapidly increased, and a project is being implemented to dam off and maintain the. western section of the lake, 'sacrificing' the eastern part (Aladin and Plotnikov, 1993). Lands exposed by the receding water levels in Lake Balkhash, apparently polluted by emissions fallout from a copper smelter and missile tests during the Cold War era, are laid bare to the prevailing winds, raising the spectre of Aral Sea-style dust storms (Bond et al., 1992).

Probably the best-known example of the creation of a dust storm resulting from the draining of a saline lake is the Aral Sea, on the border of Kaza- khstan and Uzbekistan in Central Asia. Some belief, and indeed historical evidence, suggests that the Aral Sea basin may have naturally filled and desiccated intermittently with wn-iations in climate even during historical time; for example, the memoirs of Sultan Babur of Timurids, Emperor of the Great Moguls, indicate that the Ar~LI basin was completely dry in parts of the 15th and 16th centuries A.D. (Dunin-

Barkovsky, 1977). Once the fourth largest lake on Earth in terms of surface area, the surface area of the Aral Sea has shrunk by more than 40% and its surface elevation has dropped by 10 m since 1960 (Kadukin and Klige, 1991), primarily by excessive consumptive use of water from the Amu Darya and Syr Darya rivers that flow into it (Micklin, 1988). More than one million hectares of desertified lakebed was exposed by 1990 (Sattarov et al., 1991).

Micklin (1988) summarized the huge extent and nature of dust storms generated by recession of the Aral Sea. Major dust storms have been generated from the relict surface since the mid-1970s. Twenty- nine storms, large enough to be observed from satel- lite images, occurred before 1981. Dust from the Aral Sea has been deposited over a region which extends from the Black Sea coast to the Arctic. Micklin estimated that 43 million Mg of salt are deposited as aerosols each year over an area that may be as large as 200,000 km 2. The plumes of dust from the Aral Sea are composed primarily of sodium and calcium salts, with additional constituents that include anthropogenic compounds such as pesticide residues. The human health hazard presented by these particulate episodes is significant; some studies indicated that inhalation of fine particulate matter from the Aral Sea has been responsible for marked increases in many diseases, including cancer of the respiratory tract (Micklin, 1988: Ellis, 1990: D.A. Gillette, pers. commun., 1991). The desiccation of the Aral Sea has increased the continentality of the climate in the region within 100 to 200 km of the sea, causing decreased precipitation and greater ex- tremes of temperature (Kotlyakov, 1991), such as the unusually cold winter of 1993-1994 (Zatoka, 1994). Pre-existing natural landscapes, such as floodplain forests (tugai) have disappeared, greatly reducing the biodiversity in the region (Kotlyakov, 1991).

Mitigation of the 'Aral Sea problem' will itself create impacts on other terminal lakes in the region. Among the amelioration schemes which have been proposed, channelling of irrigation drainage water into the sea from the Amu Darya basin would result in the desiccation of Sarykamysh (Sarikamish), a 2550 km 2 reservoir. Sarykamysh was formed by runoff of saline, agrochemical-laden drainage water from Uzbekistan and Turkmenistan into a naturally desiccated playa. The filling of Sarykamysh began in

216 T.E. Gil l / Geomorphology 17 (1996) 207-228

1961, the year that the Aral Sea began its precipitous decline (Micklin, 1988: Kikichev et al., 1990). If it is desiccated in order to mitigate the Aral Sea, Sarykamysh itself might become a new, albeit smaller, source of dust. The Transkarakumsky Col- lector was brought forward as another alternative strategy for confronting the problem of waste and drainage water dumped into the Amu Darya; this series of canals would transfer water more than 500 km into the Karashor Depression, which is otherwise a closed basin. Aranbaev (1994) described the Tran- skarakumsky Collector plan and expressed concerns that completion of the canal could lead to deflation of dry saline soil into dust storms.

4.4. Australia

Kappakoola Swamp is a playa in the central Eyre Peninsula, South Australia. A set of dunes atop an atypically shaped lunette on its south side was de- duced by Smith et al. (1975) to have formed as a result of clearing of mallee bushes on the crest of the lunette in 1915 or 1916. The authors suggested that other multiple lunette fields in southern Australia might have formed as a result of similar human activities. Williams (1993) noted the role of overgrazing by rabbits and livestock in breaking the surface of crusted soil in saline lake basins and making the land more susceptible to erosion.

4.5. North America

Old Wives Lake in Saskatchewan, Canada, is part of an extensive region of saline terminal lakes in the northern Great Plains. In the early 1990s, it dried completely because of the combined effects of drought and overdraft of water. Prevailing northwest winds blowing across the 180 km 2 playa led to the generation of dust plumes composed of sodium sulfate salts and clastic silts and clays, affecting the well-being of residents in communities downwind. Gomez et al. (1992) published a careful epidemiolog- ical study of the human respiratory effects of breath- ing the dust, one of the first studies of short-term health effects of alkaline playa dust. The investigators documented a statistically significant increased prevalence of cough, chronic cough, chronic wheeze,

chronic eye irritation, and chronic nasal irritation in the sample of 300 individuals exposed to the dust as compared to non-exposed individuals. They did not, however, find a difference in lung function between the dust-exposed and the control populations. Garrett (1992) described dermatitis and respiratory tract effects of exposure to industrial soda ash dusts, which would mimic acute exposures to eolian dusts from Old Wives and many other saline playas (including Owens and Texcoco).

In the western USA, examples of actual or potential playa-generated dust storms exacerbated by human activity have been reported at many locations. The northern third of the Great Salt Lake in Utah was cut off by the construction of a railroad cause- way across it in 1959, changing salinities and surface levels of the two sections (Jones et al., 1977) and leading to the deposition of a thick bed of halite in the northern arm (Eugster and Kelts, 1983). Reports expressed concern that the resulting processes could desiccate a large area of the land and cause dust storms that would render parts of the Salt Lake Valley uninhabitable (Adams, 1964). Fortunately, to date this has not happened. A number of other locations in the Great Basin and Southwest, including Surprise Valley in northeastern California, Goose Lake on the California/Oregon border, several saline lakes in Oregon (Garrett, 1992), Willcox Playa, Ari- zona (Thomas, 1963, pp. E17-E19), and several playas in Nevada (Melhorn and Trexler, 1977; Mayer, 1993), have been desiccated at least in part because of increased withdrawal of surface water or groundwater, leading to blowing sand and/or plumes of airborne salt dust. Remnants of Pleistocene Lake Lahontan have been further desiccated by anthropogenic activities: Winnemucca Lake was completely desiccated, and the level of Walker Lake has declined more than 30 m as a result of agricultural practices (Smith et al., 1989); diversion of 42% of the inflow from the Truckee River for agricultural use (Milne, 1987) led to a 19-m drop in the surface level of Pyramid Lake between 1904 and 1992 (Lebo et al., 1994). Moderate to large plumes of dust have been observed rising from the exposed playa on Pyramid Lake (N. Lancaster, pers. commun., 1995). Between 1890 and 1910, homesteaders cleared natural vegetation from the floor of the pluvial basin of Fort Rock Lake, Oregon, resulting in rapid deflation


of up to 1 m of previously stabilized volcaniclastic playa surface material (Allison, 1966).

The Mojave Dese:rt region of southeastem Cali- fornia contains many dry lakes. The Antelope Val- ley, on the urban fringe of Los Angeles, is the westemmost extension of the Mojave Desert and contains three playas (Thomas, 1963, p. E45). Fac- tors related to urbanization, land speculation, and abandonment of agriculture have led to eolian resus- pension of sediments, resulting in clouds of dust and invasion of residential areas by moving sand (Spitzer, 1993; Farber et al., 1994). Wilshire (1980) described the acceleration of wind erosion at a number of dry lakes in the Mojave, producing plumes of dust easily discernible on LANDSAT images during a Santa Ana wind event on 1 January 1973. One dust plume was derived from H~uper Dry Lake, where agricultural land use had stripped the natural plant cover from the upwind edge of the playa and replaced it with a bare surface. Off-road vehicle activity along

the edges of playas in the Mojave Desert has resulted in severe land degradation, destroying vegetation and breaking up crusted surface sediments (Fig. 3), and accelerated the potential of these locations to be regional 'point sources' of dust plumes visible from space.

Another example that may prove difficult to manage is the Salton Sea, in extreme southern California. In historic time, the Salton Sink was the completely desiccated playa of Lake Cauhilla, until it filled accidentally between 1905 and 1907. Diversion of the Colorado River for irrigation enhancement accidentally breached a canal in late 1905, and for sixteen months the river flowed into the below-sea- level sink rather than the Gulf of Califomia. The Salton Sea has remained filled and become increasingly saline, as a result of artificial inflow from agricultural drainage, storm runoff and wastewater discharge from Mexico and California (McClurg, 1994). In recent decades, attempted and abandoned

Fig. 3. Degradation of l:laya surface by off-road vehicles, Koehn Dry Lake, northwest Mojave Desert, California. (Photo courtesy H. Wilshire.)

218 TE. Gill / Geomorphology 17 (1996) 207-228

urban development of the shoreline exposed shoreland sediments extremely rich in selenium and chemical residues (Wilshire, 1980; Schroeder et al., 1988). These materials have already been deflated in storms which are of sufficient concentration to exceed Cali- fornia particulate air pollution standards, but the situation has the potential to intensify. The present level of the Salton Sea is maintained to a large extent by the drainage of excess agricultural water. U.S.A. government-mandated water conservation measures are expected to cause the lake to shrink further, and its salinity to reach 90 g 1- J by the year 2010, nearly double the present value (Black, 1983; Dexter, 1993). Conservation of valuable and scarce water resources in the Imperial Valley is necessary, but any plan which leads to a rapid desiccation of the Salton Sea could expose potentially toxic material which could be easily eroded by the wind (Gill, 1990a).

Just south of the California (U.S.A.)-Baja Cali- fornia (Mexico) border, Laguna Salada is a normally dry playa between the Salton Sea and the Gulf of California. Laguna Salada is an abundant source of

saline dust, generating carbonate-enriched soil horizons in fans and terraces downwind of the playa (Mueller and Rockwell, 1995). In the past, the region has been uninhabitable by humans because of extremely saline dust storms (Associated Press, 1993). After heavy rains in the Colorado River basin in 1993, however, the lake filled again, and several plans are under consideration to permanently recon- nect it to the Gulf of California and/or the Salton Sea (Associated Press, 1993; McClurg, 1994).

Lake Texcoco, located near the Mexico City international airport, is the remnant of a Pleistocene pluvial lake which was the terminal lake of the Valley of Mexico (Bradbury, 1989). After a series of historical floods that inundated parts of Mexico City, a project to drain the lake via a series of 'dewatering canals' began in 1910 (Ives, 1938). The complete draining of Texcoco was accomplished by the 1960s, in an attempt to convert the lakebed into an agricultural region and provide water for agricultural, industrial, and urban use (Jauregui, 1989; Goudie and Middleton, 1992; Garrett, 1992; J. Miranda, pers.

Fig. 4. Wind-erodible playa ring formed by desiccation of Mono Lake, California. Southeast edge of lake, looking south.


commun., 1994). The, formation of choking saline dust clouds from the exposed lakebed, locally referred to as 'tequezquite', has been noted since the 1930s. Ives (1938) was among the first to describe the environmental impact and decry anthropogenic desiccation of 'lakes of interior drainage'. After its total desiccation, the problem of dust from the playa of Lake Texcoco gravitated from a chronic nuisance to an acute ecological disaster. In recent years, however, the dust storms of Lake Texcoco have been successfully mitigate&

4.6. Mono Lake and Owens Lake

Mono Lake and Owens (dry) Lake, located at the edge of the Great Basin at the base of the Sierra Nevada in California, are among the largest individual sources of fugitiwe dust in North America. These lakes were part of a Pleistocene pluvial lake drainage system in which w~tters in Lake Russell (pluvial Mono Lake) flowed farough Adobe Valley into Lake Owens (pluvial Owens Lake), and then through China Lake, Searles Lake and Panamint Lake into pluvial Lake Manly (Death Valley). In historic time, Mono and Owens Lakes were the only permanent bodies of water in this system.

Mono Lake, situ~tted directly east of Yosemite National Park, is currently fed by a number of streams draining the east slope of the Sierra Nevada. As much as 65 km:' of highly erodible playa has been exposed in the Mono Basin since water diversion from its feeder streams by the Los Angeles Division of Water and Power (LADWP) began in late 1940. The surface elevation of Mono Lake has dropped by as much as 14 m, to a point where the lake contained the least water in any natural fluctua- tion of the past sew,'n centuries (Stine, 1990). The exposed playa (Fig. ,4) is composed of a mixture of sulfate-rich evaporite salts, clastic sediments and volcaniclastic materials, and is enriched in arsenic deposited by sublacustrine hydrothermal activity (Cahill and Gill, 1987; Gill, 1990b). In addition to development of dust storms from the playa, other environmental impacts at Mono Lake include mortality of pelagic biota from e'~cessive salinity, and the subse- quent effects on the prey base of aquatic birds, including potential abandonment of regionally important breeding colonies of the California gull

(Larus californicus). The drying of Mono Lake is one of the primary environmental policy battle- grounds in California; scientific research on effects of the desiccation of the lake has been cited as significant in political decisions on the future of the Mono Basin (Stine, 1991; Wiens et al., 1993).

Several reports (Kusko et al., 1981; Kusko and Cahill, 1984; Cahill et al., 1986; Cahill and Gill, 1987; Gill and Cahill, 1992a) have documented and quantified the nature, extent, and environmental impact of dust storms derived from the exposed playa of Mono Lake. When no dust was observed (about nine days in ten), the air at Mono Lake was among the 'cleanest' air in California. On days during which dust was observed (11% of all days), the 24-h mean values of suspended dust averaged 580 p~g/m 3, almost six times greater than the then-California state standard for Total Suspended Particulates (100 ~g/m3). Essentially all of the dust consisted of playa materials (Gill, 1990b); the dust events violate the California air quality standard for sulfates (Kusko and Cahill, 1984), and contain sufficient arsenic to cause an increased cancer risk in humans (O. Raabe, Appendix A to Cahill and Gill, 1987). Almost all Mono Lake dust particles are of a size approximately 11 p~m or smaller, and are, therefore, capable of causing irritation to the human respiratory tract; approximately 10% of these particles are smaller than 2.5 ~zm, and, therefore, can be inhaled deep into the lungs (Kusko and Cahill, 1984).

The deposition of alkaline dusts from the Mono Basin has the potential to eventually change the soil chemistry and structure of vegetation communities in areas downwind of the playa. Baath et al. (1992) documented the biogeochemical effects of alkaline dust on a soil profile in Finland polluted by alkaline aerosols from a steel factory; the pH of the humus layer was raised from 4.1 to 6.6, and the growth rate of soil bacteria increased by a factor of 1.8. The pH of the surface soil horizons on the east side of the Mono Basin directly downwind of the playa has increased in the last several decades (P. Kilham, pers. commun., 1986); this is quite conceivably related to the deposition of dust particles (Cahill and Gill, 1987).

Water diversions from the Owens River into the Los Angeles Aqueduct began in 1913. Since 1926, diversion of surface water and groundwater by the

220 T.E. Gill/Geomorphology 17 (1996) 207-228

Fig. 5. Dust cloud from Owens (dry) Lake filling Owens Valley and spilling over crest of Sierra Nevada (left). South end of Owens Valley, looking north. (Photo courtesy P. St-Amand.)

Los Angeles Department of Water and Power out of the Owens Valley has been near-continuous (Nadeau, 1950; Gill and Cahill, 1992b). Owens Lake, left

without inflow, soon desiccated into a 280 km 2 playa. Groundwater discharge to the dry surface from the remaining brine pool beneath the lake has

Fig. 6. Dust plume rising off southwest playa of Owens (dry) Lake and obscuring Sierra Nevada. Southeast shore of playa, looking west. (Photo courtesy L. Ashbaugh.)


allowed the deposition of effiorescent salts and salt crusts extremely vulnerable to wind erosion (Barone et al., 1981; Gill and Cahill, 1992b; Cahill et al., 1994, 1996). Dust stc,rms from the exposed lakebed began in the 1930s (Reinking et al., 1979), and have become very large. Re, sidents coined the term 'Keeler fog', named after the port town left high and dry on the east shore of the lake, for this dust which pene- trates the tiniest crevice and contaminates dwellings (Cahill et al., 1994, 1996). Ono and Cox (1990) estimated that Owens (dry) Lake produces an annual mass of PM10 (airborne particulate matter smaller than 10 txm aerodynamic diameter) approximately 2.5 times greater tha:a the annual mass of all regu- lated air pollutants from the Los Angeles air basin. The prevalence of high winds in the Owens Valley and at least 118 km 2 of wind-erodible sediments on the playa (Ono et al., 1994) have combined to make Owens (dry) Lake the single largest source of fugitive dust in the United States, with dust production estimates ranging up to 8 million Mg per year (Gill and Gillette, 1991), and PM10 concentrations mea- sured over a 2-h pe:dod exceeding 40,000 txg/m 3 (Cahill et al., 1994, 1996).

The dust plumes from Owens (dry) Lake quickly rise above the lakebe, ds to a very significant height, filling the atmospheric boundary layer and spilling through topographic gaps and over the top of the surrounding mountal:a ranges (Fig. 5). Dust plumes rise almost vertically from Owens (dry) Lake to considerable altitudes; plume depths may approach 1 km before moving more than 1 km downwind (Fig. 6). Dust from Mono Lake has been traced blowing eastward over three successive basins and ranges into Nevada (Cahill and Gill, 1987), while dust storms blowing off of Owens (dry) Lake have been traced over an area exceeding 90,000 km 2 and as far distant as Orange County, 250 km south, where the alkaline dustfall damaged the paint on cars (St. Amand et al., 1986). Dust from Owens (dry) Lake has a significant adverse environmental impact on three surrounding national parks, numerous wilder- ness areas, and designated areas of critical environmental concern. Mereover, it has resulted in economic losses of millions of dollars at the nearby China Lake military base because of suspension of operations during dust events (Cahill et al., 1994, 1996). Owens (dry) Lake was the site of a major

multinational field study (LODE, the Lake Owens Dust Experiment) in 1993, involving scientists with extensive experience in the investigation of processes generating dust in the Sahara, at the Aral Sea, and at Mono and Owens Lakes. The results of LODE are expected to provide significant insights into the processes of dust generation and transport from playas (e.g. Gillette et al., 1995, 1996; Cahill et al., 1994, 1996).

Six conditions are necessary for the development of a dust episode at Mono and Owens Lakes: saline groundwater near the surface; the precipitation of effiorescent salt crusts by the discharging groundwater; a sustained wind above a threshold velocity of approximately 11 m / s (Mono Lake) or 7 m / s (Owens); high wind shear over the flat, unobstructed playa surface; a sufficient fetch length across the playa (on average, at least 1.5 km); a supply of coarse, sand- or larger- sized clasts that help break up the efflorescent crust into finer particles capable of deflation or abrasion as dust aerosols.

The key physical mechanism which allows the generation of aerosols at Mono and Owens Lakes has been conclusively shown to be abrasion of the surface by saltation, not direct deflation of puffy efflorescences (Cahill et al., 1994, 1996). Wind tun- nel tests of pieces of the playa surface observed to be eroding showed that the crust by itself could not be directly suspended or broken up into aerosols smaller than 15 ixm under any conceivable natural wind velocities (Gillette et al., 1982). Yet the observed threshold of wind velocity for dust suspension at Owens and Mono Lake playas is characteristic of the threshold velocity for motion of sand-sized grains 100 to 200 Ixm in diameter. The saltation of larger grains along the surface of the playa chisels into the surface of the efflorescent crust and momentum transfer disaggregates and dislodges fine grains that can be suspended into the airstream. At the Mono and Owens playas, abundant sources of saltating grains exist, including broken pieces of playa crust, clastic lacustrine sediments, alluvial materials deposited by mass wasting and fluvial processes, salt weathering of larger clasts (at Owens (dry) Lake), and tephra (at Mono Lake). Several extensive sand sheets exist along the outer margins of Owens (dry) Lake, marking areas of eolian reworking of large- scale sediment deposition on former strand lines. The


3500

3000

2500

E 2000

E 1500 t~

1000 E

500

573 552

331 307

0 . I ~ I ~ - - i Inyo County Mono Imperial San Largest

County County Bemardino single value County of all 54

others

Fig. 7. Maximum total suspended particulate readings for California, 1982, by county (after Kusko and Cahill, 1984).

areas of contact between the sand sheets and saline playa crusts are the zones of most active eolian erosion (Gillette et al., 1996; Cahill et al., 1996).

Fig. 7 summarizes data for total suspended particulates in the State of California for the year 1982, showing the maximum daily concentrations of particulates by county. The highest values relate to known locations of dust-producing, anthropogenically desiccated playas. The greatest concentration of dust was found in Inyo County, in an Owens (dry) Lake dust storm; the second highest was Mono County, from Mono Lake playa dust; the third highest county (of 58 total) was Imperial County, location of the Salton Sea; and the fourth greatest value was for San Bernardino County, in the Mojave Desert.

5. Mitigation and rehabilitation of anthropogenically desiccated playas

The continued nuisance and exceedance of air quality standards for dust produced from playas, especially the Aral Sea and Owens (dry) Lake, has led to a focus by numerous agencies and research teams on ways to control or ameliorate the production of mobile sands and suspended dusts. Appropri-

ate solutions for dust mitigation or land rehabilitation at large desiccating lakes in complex geomorphic environments, such as Lake Texcoco, Owens (dry) Lake, the Aral Sea, and Poyang Lake in China, generally require the implementation of a combination of land rehabilitation techniques in different landforms surrounding the zone of desiccation (Blanco, 1950: Li Jicheng, 1990: Gill and Cahill, 1992b.)

At Mono Lake, continued water diversions have been limited by court order, and some streamflows have been returned to the lake, at least in part because of the blowing dust caused by the exposed lakebed (Wiens et al., 1993). On 28 September 1994, the State of California Water Resources Control Board ruled that LADWP must "restrict Mono Basin water exports in a manner that is intended to result in the water level of Mono Lake rising to an elevation of 1,948 m in approximately 20 years," stating among the reasons for its ruling "the higher water level will.., reduce blowing dust from presently exposed lakebed areas in order to protect health and comply with federal air quality standards" (State of California Water Resources Control Board, 1994, pp. 2-3).

At the Aral Sea, many mitigation schemes have been proposed, including the Transkarakumsky


Canal. Another plan would build a huge pump-and- pipeline system to bring water to the Aral some 2400 km from western Siberia (Micklin, 1988).

In the Great Konya Basin, a three-part land rehabilitation program wa,; successfully implemented over an area of 16,000 ha in the basin between 1962 and 1972 (Cevik and Bir]~aaan, 1986). The project zone was fenced off by barbed wire, wells were bored, and sand motion was effectively mitigated by imple- menting a combination of reed windbreak curtains and revegetation.

The desiccation of Poyang Lake in central China resulted primarily from neotectonic activity and other natural geologic processes, and led to the remobilization of sand sheets and degradation of the surrounding lands. A plan has been formulated to mitigate the desertification through the use of vegetation for sand mobilization and soil erosion control (Li Jicheng, 1990).

In the Valley of Mexico, the 'tequezquite' clouds have been mitigated to a great extent by the implementation of the Proyecto Lago Texcoco (Lake Tex- coco Project). The saline brines in the lake are actively being pumped out for soda ash production (Garrett, 1992)--just as at Owens (dry) Lake--and the lakebed is being reclaimed by planting saltgrass (Distichlis spicata). The proportion of dust events in Mexico City attributable to Lake Texcoco declined from 40% in 1971 to 0% in 1984 (Goudie and Middleton, 1992). The experience of Lake Texcoco shows that mitigation is possible: unfortunately, Mexico City has other severe air pollution problems that must be addressed.

The planting of saltgrass (Distichlis spicata), successfully used for the reclamation of Lake Texcoco, is a prime candidate tbr revegetation efforts at Owens (dry) Lake (Yau, 1993; Cahill et al., 1994). Several characteristics of Owens (dry) Lake, however, will make it much more difficult to mitigate. Owens (dry) Lake is much more saline than Lake Texcoco, and it would be economically unfeasible to pump the Owens brines dry. In addition, extreme salinities and extensive 'sandblasting' of plants have limited the success of past revegetation efforts on the playa (Fisher, 1984). Several major projects have recently been implemented at Owe:as (dry) Lake to test methods of dust mitigation; these have included flooding, sprin- kling, or chemically stabilizing parts of the playa,

and the emplacement of sand fences to build artificial dunes of impounded saltating grains (Gill and Cahill, 1992a,b; Cahill et al., 1994; Ono et al., 1994).

6. Conclusion

The playas left behind by human desiccation of Owens (dry) Lake and the Aral Sea are the largest individual sources of fugitive dust in North America and Asia. The eolian sediments produced by desertification of terminal lakes, an adjustment of the geomorphic system to sudden perturbations in water, energy, and sediment balances, have human impact (economic losses, respiratory health effects, traffic accidents, desertion of settlements), environmental impact (degradation of air quality and sensitive ecosystems), and potential geomorphological impact (changes in playa and dune surfaces, and development of eolian horizons in soil profiles). The human disturbance of terminal lakes has created small-scale simulations of the desiccation of pluvial lakes during interglacials/interpluvials, and allows us to gain a greater understanding of the geomorphic response to climatic change over what would otherwise be greater scales of space and time.

However useful desiccating playas might be for scientific investigation, the most important priority for residents of areas such as the lower Owens Valley, the zones downwind of Old Wives Lake and Lake Texcoco, the Great Konya Basin, the Aral Sea region and elsewhere, is mitigation. New types of human intervention, including land rehabilitation and/or cessation of some extant human practices in the already anthropogenically perturbed system, may be costly and not produce an immediate amelioration, but their prompt implementation is necessary. Some 45 years ago, Blanco (1950) stated, " The dust storms are only one of the many manifestations of ecological alteration or disequilibrium provoked by man in the extensive Valley of Mexico. Whatever isolated method which is adopted to try to avoid the storms of dust, without taking into account the logi- cal sequence of interrelations which exist between this molesting manifestation and its diverse causes, will show itself to constitute, in the best of cases, so much as only a brief and very costly palliative. If the


factors already mentioned in the body of this work can be summarized, other non-federal erogations to stop the dust storms and to abstract the water to the city of Mexico, after almost 50 years, will be just a fantasy.. , and through the wasted millions of those who can keep going on wasting their efforts in foolish new schemes to resolve these problems, the dust storms will go on and keep going on year after year, each time with greater intensity, as a forceful testimony of our ecological incompetence. The only definitive remedy is to revert the lake to its natural condition."

This quote refers to Lake Texcoco, but it could easily refer to any of the other examples mentioned in this report.

Acknowledgements

I thank Javier Miranda, Kyle Noderer, Rebecca Romani and Xianzhou Zeng for i assistance with translations from Spanish, Russian, French and Chi- nese, respectively. I also thank Lowell Ashbaugh, Analisa Bevan, Thomas Cahill, Deborah Elliott- Fisk, Dale Gillette, Charles Higgins, Nick Lancaster, Craig Quezada, Scott Stine, Greg Torres, Harry Walker, Howard Wilshire and an anonymous reviewer for their assistance and comments. Reconnaissance of playas in the western USA was performed while I was a Switzer Foundation Fellow in the Environmen- tal Sciences; support as an investigator of dust storms caused by desiccation of Owens and Mono Lakes was provided by contracts with the Community and Organization Research Institute, California Air Re- sources Board, California State Lands Commission, and the Sierra Nevada Ecosystem Project.

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