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Preliminary Assessment of Stability Along a Bioclimatic Gradient, North Central and Northwestern

Carlos Cordova Oklahoma State University, Stillwater, OK

Jess Porter Oklahoma State University, Stillwater, OK

Kenneth Lepper North Dakota State University

Regina Kalchgruber North Dakota State University

Gregory Scott Natural Resources Conservation Service

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Cordova, Carlos; Porter, Jess; Lepper, Kenneth; Kalchgruber, Regina; and Scott, Gregory, "Preliminary Assessment of Sand Dune Stability Along a Bioclimatic Gradient, North Central and Northwestern Oklahoma" (2005). Great Plains Research: A Journal of Natural and Social Sciences. 783. https://digitalcommons.unl.edu/greatplainsresearch/783

This Article is brought to you for free and open access by the Great Plains Studies, Center for at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Great Plains Research: A Journal of Natural and Social Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Great Plains Research 15 (Fall 2005):227-49 © Copyright by the Center for Great Plains Studies, University of Nebraska-Lincoln

PRELIMINARY ASSESSMENT OF SAND DUNE STABILITY ALONG A BIOCLIMATIC GRADIENT, NORTH-CENTRAL AND NORTHWESTERN OKLAHOMA

Carlos E. Cordova and Jess C. Porter

Department of Geography Oklahoma State University Stillwater, OK 74078 [email protected]

Kenneth Lepper

Department of Geosciences North Dakota State University, Stevens Hall Fargo, ND 58105

Regina Kalchgruber

Department of Physics Oklahoma State University Stillwater, OK 74078

Gregory Scott

Natural Resources Conservation Service Stillwater Soil Survey Office, 100 USDA, Suite 206 Stillwater, OK 77074

ABSTRACT-Sandhills of eolian origin and currently active in Okla­ homa are located mainly on the northern side of the main . Their longi­ tudinal distribution spans a gradient of annual precipitation ranging from 914 mm in the east to 403 mm in the west. types along this gradient include cross-timbers woodlands in the east and sand-sage and short grasses in the west. The information presented here is a preliminary assessment of sand dune dynamics and morphology, soils, and vegetation as the basis for an ongoing study on past and present processes of sand dune stability. For this purpose, six areas along the east-west precipitation gradient were selected to evaluate potential sources of information. Pedostratigraphic data were used to reconstruct prehistoric landscape-change events and sequential aerial photographs were used to reconstruct modern processes affecting sand dune stability in the context of climate change and human agency.

Key Words: bioclimatic gradient, Oklahoma, sand dunes, soils, vegetation, wind erosion (eolian erosion)

227 228 Great Plains Research Vol. 15 No.2, 2005

Introduction

Sand dunes in Oklahoma occur mainly along the large valleys that cross the state from west to east, a pattern common across the Great Plains. Stabilized and active sand dunes in Oklahoma cover 29,000 km2, approximately 15% of the state's total area, of which only 20.7 km2 are active sand dunes. Sta­ bilized dunes are fixed by different types of vegetation, including cross-timbers woodland in the east and sand-sage grassland in the west (Fig. 1). Although the current surface area of active dunes is minimal, evidence suggests that sand dune activity during 19th- and 20th-century droughts was more widespread than today (Melton 1940; Muhs and Holliday 1995; Scott 1999). The most recent period of widespread dune activity documented by aerial photography occurred at the end of the 1930s. During the subsequent decades, large tracts of sand dunes have been stabilized as annual precipitation increased (Rhinewald 2005). Low precipitation, high evapotranspiration, strong winds, and sparse veg­ etation cover were the primary causes for sand dune activity in the sand-sage grassland areas of during long-term drought periods such as the 1930s. In contrast, the cross-timbers woodlands, which are the westernmost woods of North America's eastern deciduous forests have been more effective in maintaining sand dune stability in eastern and east- (Fig. 2). This study is a preliminary assessment of sand dune stability along an east­ west precipitation gradient. Vegetative cover, soils, morphology, and location with respect to river channels are the main factors considered in examining sand dune stability in six study areas along the precipitation gradient in north-central and northwestern Oklahoma (Figs. 1 and 2). This study represents the initial investi­ gation in a long-term research project that attempts to elucidate pertinent variables of sand dune stabilization in prehistoric and historic periods.

Methods

The initial step of this research involved mapping the areas of eolian sand in Oklahoma (Fig. 1). The information on this map is based on state soil survey maps (STATSGO); maps published by Kitts (1959, 1965), Fay (1962, 1965), Fay and Hart (1978), and Stanley et al. (2002a, 2002b) under the sponsorship of the Oklahoma Geological Survey; and numerous local studies by Blair (1975), Meyer (1975), Nayyeri (1979), Robbins (1979), Brady (1989), and Scott (1999). The areas of sand dunes were further classified into categories based on their position in the river valleys and the type of stabilizing vegetation. In the process of mapping eolian sand areas, we identified seven spots with presently active dunes (triangles in Fig. 1). 103' W 1020 101' 100' 99' 98' 97' 96' 95'W I I I I I I I I I [ 42" 16" 16" 18" 26" 28" 30" 32" 34" 36" 36"40" s· -37'N S· ....po '< ~ '" '"~ '" - 36' ~ i._~t U U - g ,- ,--r ,--, '-J ,--, 02001080110_ a, CI:l po ::l Active dunes 0- A Dune fields larger than 20 ha o C ::l Stable dunes and sand sheets ~ CI:l ~ With woodlands ~ With sand sage grassland ) on fluvial terraces g ..... ~ With Shinnery oak grassland '<

~ With Shinnery oak grassland or cross timbers ~ With sand sage grasses ) 00 """,,, G23"" '" Scattered lunettes Other STUDY AREAS J. Cimarron <..u.., Escarpments 2. Beaver Precipitation 3. Woodward-Harper 4. Woods (:})':I Severe and widespread ! Annual precipitation in inches and millimeters 5 . Major 24" o Less severe and localized (610) 6 . Logan-Payne tv ~ Figure 1. Classification of stabilized sand dunes and sand sheets in Oklahoma based on vegetation cover and location with respect to the main river valleys. The map also shows the location of the main active sand dune fields and the location of the six areas referred to in this research. 230 Great Plains Research Vol. 15 No.2, 2005

It m 103"W 102' 101' 100' 99' 98' 97' 96' I I I I I I I I 5000 1500

1400 ~ 1300 4000 ApproXimate 1200 Area """""""1 ~ westem limit of Elevation 1100 Cimarron Cross TImbers 1000 Area 2- 3000 900 800 700 ~ 2000 600 , Quercus marilandica (Blackjack oak) Area~~..oQ m~~ __ Area 6- 500 and/or Q. stellata (Post oak) Woodward~ A~~a~ Logan-Payne 400 Harper Woods Area 5 - I ,J Grasses 1000 300 Major 200 IJ Artemisia spp. (Sagebrush) ~J'.5' CLIMATE 1000

Precipitation (mm).. 800 Potential Evapotranspiration (mm). 600

400

30 Wind % • (percentage of time wind 20 speed is above the threshold to move sand) 10

Cities

EOLIAN ACTIVITY .. Dune fields Present ...... Blowouts and floodplain HistOric ...... • ndge dunes SOIL SERIES Dalhart·Vona Alfisols and AridisOIS-----;:;;;;;;;;:;;;-:===~ ______Tivoli Entisols

Jester Entisols --G:;d;;t:i8;;U;;;------­Goodnight Entisols Eda Alfisols and Entisols ------­ Devol Alfisols _====:-­ Nobscot alfisols Efaula-Dougherty Alfisols Konawa alfisols ------

Figure 2. Bioclimatic characteristics of the stabilized sand dune fields along a longitudi­ nal profile in north-central and northwestern Oklahoma.

Six focus areas were selected for the purpose of a general assessment of geomorphic and biogeographic variations along the east-west precipitation gradient (Fig. 1; Table 1). Pedostratigraphic description and dating were carried out only in areas 3 and 5 (Fig. 3). The objective of stratigraphic work in these two areas was exploratory. Therefore, we did not attempt to establish a regional chronology, but to explore the potential of pedostratigraphy for reconstructing processes of sand dune activity and stability in the past. Optically stimulated luminescence (OSL) and accelerator mass spectrom­ etry (AMS) 14C radiocarbon dating are the techniques used to determine the age of sections in Areas 3 and 5. OSL and AMS 14C ages for sections in Area 3 are Preliminary Assessment of Sand Dune Stability 231

TABLE 1

RECENT EOLIAN ACTIVITY IN THE FOCUS AREAS OF THIS STUDY

Wind data Sand station, mobility Area Counties (city) index (M)* Eolian activity Vegetation Cimarron 43.0 1930s and sporadic blow- Sand-sage (Boise City) outs afterwards. Some grassland current activity in agricultural lands 2 Beaver 46.4 Throughout the 20th Sand-sage (Beaver) century, with remarkable grassland activity in the 1930s. Parabolic dunes are stabi- lized. Sporadic blowouts occur. Transverse dunes are still active, due to ATVriding. 3 Woodward- 29.6 1930s blowout. Sporadic Sand-sage Harper current blowout activity. grassland (Woodward) 4 Woods 31.1 Mainly blowout and Cross- (Freedom) transverse, active through timbers and the 20th century. Ridge Sand plum dune activity near fiood- plain. ATV riding keeps a large area of transverse dunes active. Parabolic dunes are stabilized. 5 Major 20.5 Sporadic blowout activity Post and (Fairview) on old dunes and sand blackjack drifting near the river oak cross- channel during the 20th timbers with century. None visible at open areas present. of sand plum shrubs 6 Logan-Payne 13.9 None reported. Post and (Guthrie) blackjack oak cross- timbers

* Based on Lancaster (1988) N LL l.;.) v~~ ., l l L \. l N Active sand dunes L L L 3 :/M ' ' ~ ~~ ; L L L LL <$i, . , I L L L L o Stabilized sand dunes and sand sheets e Ie ~ el Terrace deposits : ' E,' ' / '~ I,: ,::,:,:1 Streambed and floodplain deposits r, .~~\t· J : . , E ' ~ River channel

o Creeks. tributary streams . ,I' E-T' , ,T' E ~ Gypsum escarpment WOO 1 .E-T ' ' eI - , a ,'E ~T @ N' L . ' • ~ 'T Soil series T ' ::s1 E-l' E\Ms', N' ~ 'J: , L Reservoir o Devol S' en \.. - c. Hanor E Eda ~ t en N Nobscot Buckminster (1) Towns Q T Tivoli Ames 8::r Main highways -e- J Jester Hajek 1 and 2 /[t/, ~ 2 3 4 mi 5 mi >-' VI Figure 3, Areas 3 (Woodward-Harper), 4 (Woods), and 5 (Major), and locations of studied stratigraphic sections. ~ N oN o VI Preliminary Assessment of Sand Dune Stability 233

TABLE 2

AMS 14C and OSL AGES FOR SECTIONS IN AREA 3

AMS 14C in bulk

Lab Section, depth Conventional 14C number (cm) age (yrBP) 2-0 calibrated result Beta- WMA-l 2900 ±40 BP 3160-2920 BP 189465 60-70 Beta- WMA-l 5690 ± 50 BP 6630-6390 BP 189466 140-150

Beta- WOO-l 103.2 + 0.5 pMC* Post 1950 AD 189467 48-52

* percent modern carbon

OSL

Section depth Dose Dose rate Age (cm) (yr BP)

WOO-l 0.61 ± 0.057 2.86 ± 0.14 213 ± 23 70cm

reported in Table 2. Ages obtained from sections in Area 5 are reported in Lepper and Scott (2002, in press). We made a preliminary assessment of active sand dunes in Little State Park (Area 4) using vertical aerial photography to compare the extent of active sand dunes in 1937 and 1995. Changes in this area are now being studied and compared to climatic trends for the decades following the 1930s drought. Normal precipitation and temperature data for the period 1974-2003 were obtained from the National Climatological Data Center (NCDC) and were used for most locations. Data for assessing climate trends at Little Sahara State Park covered the period 1928-2003. Wind-rose data for the period 1994-2003 were obtained from the nearest Mesonet station located at Freedom, Woods County, Oklahoma. For the purpose of evaluating sand dune activity at Little Sahara State Park, sand drift potential (DP), resultant drift potential (RDP), and resultant drift direction (RDD) were calculated and plotted following the methodology 234 Great Plains Research Vol. 15 No.2, 2005 proposed by Fryberger (1979). The sand mobility index (M) (Lancaster 1998) was calculated for the six selected areas along the bioclimatic gradient (Table 1). Lancaster's sand mobility index evaluates sand mobility as a function of the percentage of time that wind is above the threshold for sand transport (W) and the ratio of precipitation to potential evapotranspiration (P:PE). Thus, the index is obtained as follows: M = W / P:PE. Subsequently, M values were discussed in the context of values from other locations in the Great Plains (Muhs and Maat 1993; Muhs and Holliday 1995).

Geomorphic Settings and Sand Sources

Active and stable sand dune fields in Oklahoma are primarily located on the northern side of the major rivers. Melton (1940) attributed this locational pattern to the predominantly southerly wind direction, assuming that most originated from the alluvial deposits in the riverbed. Although this is true for the ridge dunes along the floodplain, later studies determined that the pre­ dominant location on the north sides is rather a result of sand source in Pleisto­ cene and Early- to Middle Holocene alluvial deposits, most of which are located on terraces along the northern riverbanks (Blair 1975; Meyer 1975; Brady 1989; Scott 1999; Lepper and Scott 2002). The preponderance of Pleistocene alluvial terraces on the northern banks responds to the long-term southward migration of the main river valleys (Madole et al. 1991), which is preconditioned by the regional southwestward dip of the Permian formations (see geologic profiles in Fay 1962). In the , the location of sand dune fields with respect to the main river channels varies. Along the Beaver River in Cimarron County, sand dunes occur on both sides. Here the sand originates from channel alluvium and weathered material from the Cretaceous sandstones along the valley (Rothrock 1925; Schoff 1943). Downstream, in and Beaver counties, sand dune fields predominantly occur on the north bank. Along the Cimarron River in Cimarron (Oklahoma), Baca (Colorado), and Morton (Kansas) counties, sand dunes fields occur primarily on the south bank, on the Pleistocene river terraces. Another common eolian sand setting is on uplands, especially along the divides between the North Canadian, Canadian, Washita, and North Fork rivers (Fig. 1). Their origin is still not clear, but some sand dunes and sand sheets seem to occupy high terraces of Lower and Middle Pleistocene paleochannels of the Canadian, North Canadian, and Cimarron rivers (Fay 1959, 1962). Therefore, the likely source of upland sands is the alluvial deposits of high Pleistocene ter­ races. On the basis of the age of the Lava Creek B Ash, Ward and Carter (1998, Preliminary Assessment of Sand Dune Stability 235

1999) determined that some high terraces occupying the uppermost areas of divides between the rivers are older than 0.61 million years. In Beaver County, in the Panhandle of Oklahoma, upland terraces seem to have originated from older eolian deposits corresponding to an unnamed geological unit associated with the Pleistocene Black Water Draw Formation of New Mexico and the Texas Panhandle (Reeves 1976; Stanley et al. 2002a). Upland dunes in New Mexico and Texas originated from the sandy facies of the Pleistocene Black Water Draw Formation (Holliday 1989a). However, Pleisto­ cene eolian deposits overlying the Ogallala Formation in western Oklahoma have not been thoroughly studied, dated, described, or formally defined. This makes it difficult to establish direct correlation with similar deposits in other areas of the Southern Great Plains.

Sand Dune Generations and Paleodata

Previous studies on sand dune deposits in Oklahoma have classified the different generations of sand dunes based on the soil series assigned to them by state soil surveys (Brady 1989; Scott 1999). Thus, soil series designations (Jest­ er, Tivoli, Eda, etc.) are merely divisions of dunes by surface soil development and position within the river valley (Fig. 3). We are using these designations provisionally as a guide for our research. Future mineralogical, sedimentologi­ cal, and pedostratigraphic studies will refine the chronological aspects of dune formation and will help us redefine dune-generation classification. Jester dunes are sand ridges paralleling the main channel of the North Canadian and Cimarron rivers. Evolution of the Cimarron River from a braided stream to a meandering channel and the subsequent rapid growth of vegetation on its floodplain (Schumm and Lichty 1963) have reduced the amount of avail­ able sand for ridge-dune formation along floodplain margins. Therefore, Jester dunes today are becoming more stable. Tivoli dunes have parabolic and compound-parabolic shapes and are situated in the lower river terraces, just above the Jester dunes. The majority of these dunes seem to have formed from blowouts carved on older dunes. Subsequently, the dune around the blowout develops into a parabolic dune that moves forward and eventu­ ally coalesces with other parabolic dunes to form a compound parabolic dune. They are characterized mainly by entisols and a cover of sand-sage grassland. Eda, Devol, and Nobscot dunes are found on higher terraces where they form rolling sand plains. The natural vegetation on these dunes is cross-timbers, and soils are often alfisols. However, large tracts of these woodlands have been turned into pastures and cultivated fields. Toward the eastern part of the gradient, 236 Great Plains Research Vol. 15 No.2, 2005 the Konawa and Dougherty dunes appear to be equivalent in age with the Eda and Devol dunes. The Otero and Vona dunes occur only in the Panhandle of Oklahoma. Otero dunes are parabolic, compound parabolic, and hairpin parabolic dunes of relatively recent age, possibly the past millennium. Vona dunes are gently rolling hills formed from older dunes of unknown age and are located on Pleis­ tocene higher terraces. Unfortunately, knowledge of Quaternary alluvium and eolian sands in Oklahoma is scant. Pleistocene sand dune deposits have been reported on the highest terraces and interftuvial surfaces between the major rivers. Thurmond and Wyckoff (1996, 1998) obtained dates between 9371 ± 97 and 25,970 ± 270 14C yr BP from soils formed in sand dune deposits of the Dempsey Divide, lo­ cated between the Washita and North Fork rivers (Fig. 1). However, the bulk of eolian deposits along the main river valleys have been assigned a Holocene age through relative dating (Blair 1975; Meyer 1975; Brady 1989). Because of the paucity of chronological data, our research team began a pilot project focusing on obtaining numerical dates from selected locations in the stabilized dunes in Areas 3 and 5 (Fig. 3). On the northern side of the North near Woodward, AMS 14C and OSL dates from the WMA-l section indicate that the Eda dunes began to form sometime around 6000 yr BP (Fig. 4). The date obtained from the A/C soil horizon at WMA-l section (6630- 6390 calculated yr BP) corresponds to a late phase of destabilization associated with the dry event recorded in the Great Plains approximately between 6000 and 4000 yr BP (Dean et al. 1996). This dry event destabilized dunes and sand sheets in Texas and New Mexico (Holliday 1989b; Muhs and Holliday 2001), southern Kansas (Arbogast and Johnson 1998), and eastern Colorado (Forman et al. 1992, 1995). A date obtained from another buried A/C horizon at 60-70 cm suggests a period of stability around 3160-2190 calculated yr BP (Fig. 4). Due to the lack of dated sections in this region, it is not clear if this is a period of stability that had a broad regional extent. However, this period of stability may correlate with soil formation dated to 2300 yr BP in sand dunes in south-central Kansas (Arbogast 1996). Near the WMA-l section, an OSL date from a Tivoli dune at location WOO-l dune provided a recent date (213 ± 23 yr BP) (Fig. 4). More recent dune activity in the area has been documented, as shown by the accumulation of sand during the 20th century on top of section WOO-I. Numerous cases of young sand deposits like this one are associated with sand blowouts formed in recent decades. At the Jake Bluff Site, located about 1.5 km east of the WOO-l section, a sand deposit similar to the Tivoli dunes is found on top of the stratigraphic ""0 >-1 C1 Section WMA-1 ~ 50 t----=-I_ 3160-2920 3' Most recent eolian activity s'po >-1 and road waste '-< ~100 ;J> V> V> (1) V> V> 3 (1) Edadune ~ g "--. g, C/J po ;:s P- O s:: ;:s (1) 0, , C/Jp; ~ g: ~post1950 ;:;: ~ 50 1 Ab1 "'-.. Section WOO-1 '-< AC 213.:!:.23 20th century r, 5 METERS I I I I 100 eolian sand o Cal AMS "c Years BP • OSL years BP

Fence t ~TIvoIId_ ------N W B:;;~~~ . ---.J ~~~7 . _____ ~

Figure 4, Stratigraphic profiles in Area 3 (Woodward-Harper), See Figure 3 for locations, AMS 14e dates reported are 2-() calibrated, 238 Great Plains Research Vol. 15 No.2, 2005

sequence. A soil below this sand was dated 500 ± 40 14C yr BP (Bement and Carter 2004). This suggests that the period of activity that formed the Tivoli dunes in this area occurred between ca. 500 and 200 yr BP. In the dune fields of eastern Major County, south of Ames, AMS 14C and OSL dates illustrate cycles of sand dune and stability (i.e., soil formation) (Fig. 5). The last period of major dune activity in this area ended ca. 800-700 yr BP, which correlates with the dry event identified in north and central Oklahoma by Hall (1982) and Hall and Lintz (1984). However, activity around 200 yr BP, as in the WOO-I section, was not recorded here. It is possible that the stabilization period did not occur at this location or was not widespread enough to become evident in the stratigraphic record.

Sand Dunes, Soils, and Vegetation along a Bioclimatic Gradient

Although most dunes along the bioclimatic gradient are stabilized, the degrees of sand surface stability vary in relation to vegetation, soil development, and recent eolian activity. The east-west gradient of precipitation encompasses 583 km (approximately 364 miles), ranging from 914 mm (36 in.) to 403 mm (16 in.) (Fig. 1). Accordingly, stabilized dunes in the eastern half of the bioclimatic transect are presently covered by cross-timbers vegetation growing on alfisols of the Nobscot, Eufaula, and Konawa soil series (Fig. 2). Dune fields in the western half of the study area are covered by sand-sage grasslands growing on entisols of the Vona, Jester, and Tivoli soil series, or on alfisols of the Eda and Devol soil series. The values of potential available moisture, obtained by subtracting poten­ tial evapotranspiration from precipitation, show a higher moisture deficit in the sand dune areas of the western half of the transect. This deficit is manifest in a less dense vegetation cover that makes soils more susceptible to wind erosion during the high percentages of time in which winds are above the speed to move sands. When these values are combined into Lancaster's sand mobility index (M), it is evident that dune destabilization is more likely to occur in the western areas (Table 1). Conversely, the surplus of moisture in the eastern areas allows for conditions conducive to sand stability. These conditions may have prevailed for centuries to millennia, allowi ng woodlands to become established on former sand dunes. The difference between western and eastern dune fields in terms of stability is portrayed by the examples in Figure 6. The western dune fields of Area 3 (Woodward-Harper) are in the process of stabilization under scattered vegetation patches of herbs and shrubs. The moisture deficiency created by TOP OF DUNE '"0 o A A A A A S~· 5' ....po 100 I,",v '-<

AC AC AC A C 00>- 00 (1) 00 200 a00 g(1) C g, en 300 po ::I Ab P- E C 'OVriO-1540 • 828't58 (.) o C 1= • 769t60 Btb ::I 400 (1) .865t79 .807t60 • 871-t68 '" .....en r-Ue- • 82yt71 Bcb -ue- po 01275-1070 Ab r-ue- g, i"DU. A.MES • 3330't230 500 Iv I"~'~ ~ -ue- Bb 3Ab HAJEK 2 3AC 600 c- A Horizon name 3C Btb -ue- OSL years BP 012800-11950 4Ab • 700 O Cal AMS 14C Years BP c- HAJEK 1 4Btb -ue- Unconformity BUCKMINSTER SITE NAME 800 I- tv HANO R VJ \0

Figure 5. Stratigraphy, radiocarbon, and OSL ages for a dune field in Area 5 (Major). See Figure 3 for locations. AMS 14C dates reported are 2-0" calibrated. 240 Great Plains Research Vol. 15 No.2, 2005

Area 3 Woodward-Harper

Blowout dune Stabilized parabolic dune

Late Holocene eolian sand

Older Holocene eolian sand

Area 6 Logan-Payne

Hnlln"'>n<> eolian sand

Late Pleistocene Perkins Alluvium

Permian red beds

Key to vegetation Ulmus Quercus Rhus amertcana martlandlca Artemisia aromatica filifolia ! ...... ~~~~;f"=-... • •... t 1... Yucca Prunus Induced Celtis Quercus glauca angustifolia pastures laevigata stellata Key to soil series T Tivoli E Eda Ko-Do

Figure 6. Models of sand dune morphology and vegetation in Areas 3 and 6.

low precipitation and high evapotranspiration results in a sparse vegetation of short grasses, principal\y sideoats grama (Bouteloua curtipendula) and buffalo grass (Buchloe dactyloides), tall grasses such as switchgrass (Panicum virga­ tum), sagebrush (Artemisia filifolia), and yucca (Yucca glauca). Shrubs and small trees such as sand plum (Prunus angustifolia) and lemon sumac (Rhus aromatica) colonize the leeward sides of the dunes. Eastern redcedar (Junipe­ rus virginiana) is not uncommon, but it does not dominate among the shrubs. Soils in the Tivoli dunes consist mainly of entisols (e.g., Typic and Thermic Preliminary Assessment of Sand Dune Stability 241

Ustipsamments). Strong winds in this area act rapidly to mobilize sand when vegetation fails to protect sand dune surfaces. The process of sand dune stabili­ zation in this area can be disrupted by occasional formation of blowout dunes. In Area 6 (Logan-Payne) sand dunes were stabilized in prehistoric times, as early-19th-century explorers had reported that these areas were covered with dense oak vegetation (i.e., the cross-timbers) on the northern bank of the Cimar­ ron River (Irving 1956; Graustein 1967). Although no numerical data exist, the development of alfisols (e.g., Thermic and Ustic Haplustalfs; Thermic and Arenic Haplustalfs) with relatively well-developed Bt horizons suggests a longer period of stability, possibly on the order of millennia. Soils here support a cover of cross-timber woodlands, although some are cultivated or used as pastures. The most typical trees of the sandhill cross-timbers in this area are post oak (Quercus stellata), blackjack oak (Q. marilandica), southern hackberry (Celtis laevigata), and American elm (Ulmus americana). Higher precipitation levels and relatively lower potential evapotranspiration help maintain stability by providing a more dense vegetation cover than is evident in the western study areas (Table 1).

Are Dunes Becoming Stabilized?

Sand dunes in Oklahoma are largely inactive, according to the models pro­ duced by plotting time that wind is above the threshold to move sand (W) and the precipitation-potential evapotranspiration ratio (P:PE) (Fig. 7). Even areas with currently active dunes such as Little Sahara and Beaver Dunes state parks (study areas 2 and 4) fall within the inactive class. This suggests that nonclimatic factors playa significant role in keeping tracts of sand dunes active. In order to test this assumption, Rhinewald (2005) reconstructed the process of landscape change in Little Sahara and Beaver Dunes state parks during the last two-thirds of the 20th century using sequential aerial photography. The reconstruction shows that the process of sand dune stabilization after the 1930s parallels the slight increase in annual precipitation, and that most of the areas of current sand dune activity cor­ respond to areas of major sand dune vegetation disturbance, particularly through ATV (all-terrain vehicle) riding. Nonetheless, despite human disturbance, active sand dune areas in Little Sahara State Park at present have declined to 75% of their total area in 1937. In Beaver Dunes State Park, the area of active dunes is only 12% of their total area in 1941 (Rhinewald 2005). The case of Little Sahara State Park is perhaps the most interesting re­ garding factors of sand dune stabilization since the Dust Bowl years (Fig. 8). Annual and decadal precipitation and potential evapotranspiration patterns in the past 75 years show a slight increase in moisture (Fig. 9; Table 3). Despite 242 Great Plains Research Vol. 15 No.2, 2005

(A) 100~------,------~

Fully 75 active

-~ :=~50

25 •5. 6

0.00 0.25 0.50 0.75 1.00 PIPE (8) 100

Largely inactive 75

(l) (l) > > o u 008 00 t5 t1l o 0 0 0 -e;:::: 50 t1l >, - oa:lO c9 0 0 ~ ~o 0 qJ, - "S .... 00 ~o 0 U. t1l 00 -l o g 00 9 :> o 0.0 0 25 0 •2 1 4•• 3 0 •5. 6

I I I I 0.00 0.25 0.50 0.75 1.00 PIPE

Figure 7. Plots showing the variables of Lancaster's sand mobility index, the amount of time wind is above the threshold velocity for moving medium sand (W), versus the ratio of precipitation (P) to potential evapotranspiration (PE) for locations in this study (squares) and other locations in the Great Plains (dots) according to Muhs and Maat (1993). (A) Dune activity classes by Lancaster (1988). (B) Modification of (A) showing dune activity classes based on P:PE values based on Muhs and Maat (1993) and Muhs and Holliday (1995). Preliminary Assessment of Sand Dune Stability 243

DP 575 1 kilometer RDP 197 RDPIDP 0.34 1 mile

Flp Floodplain J Jester dunes (stabilized in the late 20th century)

TS 1 Dunes stabilized before 1937

TS2 Dunes stabilized after 1937 T Tivoli dunes (long stabilized)

Figure 8. Little Sahara State Park dunes. Stabilized dunes and extent of sand dunes in 1937 and 1995. The photograph was taken in 1995. 244 Great Plains Research Vol. 15 No.2, 2005

1300 r------, 1200 1100 ...... ANNUAl. .. POTENTIAL EVAPOTRANSPIRATION :i 1000 .~ _. ---." ---"~ ------_.. _"------, -" ~ ------""------:E 900 Z 800 o 700 ~ 600 '"'a::: 500 (j w 400 0:: Do 300 200 100 0 ~"J ,,~W' ,,~

Figure 9. Annual precipitation, annual potential evapotranspiration, and March-April pre­ cipitation for 1928-2003, registered at Waynoka, OK, north of Little Sahara State Park. dry periods in the 1950s and 1960s, mean annual precipitation tends to increase. The most noticeable changes since the mid-1970s are a slight increase in an­ nual rainfall and relatively moderate annual potential evapotranspiration rates compared to previous decades. Averages for March and April precipitation for each year were also plotted and compared with annual values because the spring season is the most critical time of eolian activity during the year in the Southern Great Plains (Holliday 1991; Stout 2001). A slight increase in March-April pre­ cipitation is observed since the late 1960s (Fig. 9; Table 3). This suggests that moisture is becoming available for grasses and other annual herbs at the time when winds are the strongest. Therefore, minimal increases in moisture during the past three decades have led to the reduction of most active dune fields as they become stabilized with grasses, sage, and shrubs. In view of this potential scenario, our team is monitoring the seasonal development of vegetation in rela­ tion to precipitation. However, at this writing, the first annual cycle of seasonal data is not yet available. Another interesting pattern observed at Little Sahara State Park is the relatively rapid stabilization in areas where ATV riding is not allowed. Some of these areas had fully active sand dunes in 1937, but now they are vegetated (Fig. 8). Vegetation cover consists mainly of a cover of sages, tall and short grasses, yuccas, and sand plum shrubs. Although sand dune stabilization has been increasing in recent decades, the question of the fate of the dunes in a major drought still lingers. Our current research aims to collect data to create a predictive model of sand dune destabi­ lization in the event of 1930s-magnitude drought. Preliminary Assessment of Sand Dune Stability 245

TABLE 3

DECADAL AVERAGES OF ANNUAL PRECIPITATION, POTENTIAL EVAPOTRANSPIRATION, AND MARCH-APRIL PRECIPITATION AT WAYNOKA STATION, NORTH OF LITTLE SAHARA STATE PARK

Number Annual Annual potential March-April of years of precipitation evapotranspiration precipitation precipitation Decade (mm) (mm) (mm) above average* 1931-40 622.3 916.9 99.1 4

1941-50 645.2 796.6 101.6 4

1951-60 609.6 916.9 94.0 4

1961-70 640.1 825.5 86.4 5

1971-80 663.0 901.7 106.7 5

1981-90 617.2 726.4 111.8 6

1991- 668.0 873.7 129.5 8 2000

*Mean annual precipitation (1928-2003) = 655 mm

Conclusions

Sand dunes in north-central and northwestern Oklahoma are located mainly where the sources of sands are Pleistocene and Early Holocene alluvial deposits. This setting corresponds to alluvial terraces located mainly on the northern side of the large rivers. The oldest dunes are located on the highest river terraces, where preliminary dates show that they formed during the Early and Middle Holocene. The most recent dunes are located in the lower terraces and along the main channels. They include dunes ofthe Tivoli soil series which, according to preliminary chronometric data, formed during the last millen­ nium. Sequential aerial photography shows that the youngest parts of the Tivoli soil series (TS] and TS) and the ridge dunes along the river channels (Jester soil series) became stabilized during the second half of the 20th century as is seen in the area around Little Sahara State Park (Fig. 8). Our preliminary assessment suggests that sand dune stability patterns are tied to bioclimatic variations along an east-west precipitation gradient. In the east, 246 Great Plains Research Vol. 15 No.2, 2005 sand dune stabilization has been more effective for the development of oak woods of the cross-timbers woodlands. No historic dune activity is reported in this area of Oklahoma, suggesting that sand surfaces are fully stabilized. Even though some cross-timbers areas have been cleared for cultivation and other economic activities, there is no destabilization threat, particularly under the current climatic conditions. Most of the historic and current dune activity, however, has occurred west of the 98th meridian, where vegetation consists of sand-sage grassland and scattered shrubs. In these areas, frequent strong winds and low P:PE ratios allow destabilization, particularly through human disturbance. The areas of sand dune activity have decreased during the second half of the 20th century due to a slight increase in precipitation, particularly during the early spring, when the strongest winds occur. Active dunes persist mainly in areas where ATV riding occurs, as is the case in Little Sahara State Park.

References

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