Dune field reactivation from blowouts: Sevier Desert, UT, USA

Aeolian Research 11 (2013) 75–84

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Dune field reactivation from blowouts: Sevier Desert, UT, USA Thomas E. Barchyn a,⇑, Chris H. Hugenholtz b a b

Department of Geography, University of Lethbridge, Lethbridge, Alberta, Canada Department of Geography, University of Calgary, Calgary, Alberta, Canada

a r t i c l e

i n f o

Article history: Received 26 April 2013 Revised 26 August 2013 Accepted 27 August 2013 Available online 26 September 2013 Keywords: Aeolian geomorphology Dune activity Desertification Dune field reactivation

a b s t r a c t Dune field reactivation (a shift from vegetated to unvegetated state) has important economic, social, and environmental implications. In some settings reactivation is desired to preserve environmental values, but in arid regions reactivation is typically a form of land degradation. Little is known about reactivation due to a lack of published records, making modeling and prediction difficult. Here we detail dune reactivations from blowout expansion in the Sevier Desert, Utah, USA. We use historical aerial photographs and satellite imagery to track the transition from stable, vegetated dunes to actively migrating sediment in 3 locations. We outline a reactivation sequence: (i) disturbance breaches vegetation and exposes sediment, then (ii) creates a blowout with a deposition apron that (iii) advances downwind with a slipface or as a sand sheet. Most deposition aprons are not colonized by vegetation and are actively migrating. To explore causes we examine local sand flux, climate data, and stream flow. Based on available data the best explanation we can provide is that some combination of anthropogenic disturbance and climate may be responsible for the reactivations. Together, these examples provide a rare glimpse of dune field reactivation from blowouts, revealing the timescales, behaviour, and morphodynamics of devegetating dune fields. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction While the vast active dune fields of Earth are impressive manifestations of wind-driven sediment transport, there are equally vast vegetated dune fields (e.g., Muhs and Maat, 1993; Mason et al., 2004; Hugenholtz et al., 2010; Wolfe et al., 2011; Kiss et al., 2012; Zárate and Tripaldi, 2012). Hidden under forests or grasslands, vegetated (stabilized) dune fields have transitioned from active to stable (and vice versa) throughout global climate changes in the Quaternary (e.g., Miao et al., 2007). Sensitivity to climate suggests that the vast reservoirs of sediment in presentlyvegetated dune fields may become active in the future as climate changes (e.g., Thomas et al., 2005; IPCC, 2007). A shift from vegetated to active is a major land surface change. The landscape becomes inhospitable to vegetation and in cases migrating dune forms limit permanent inhabitation with parallel economic, social, and environmental implications. Dune field reactivation is a form of desertification (D’Odorico et al., 2013). While in most regions of the world desertification is considered a form of land degradation, in coastal Europe some dune fields are artificially reactivated with the goal of preserving habitat for endemic

⇑ Corresponding author. Address: Department of Geography, University of Lethbridge, 4401 University Drive, Lethbridge, Alberta T1K 3M4, Lethbridge, Alberta, Canada. Tel.: +1 403 332 4043. E-mail address: [email protected] (T.E. Barchyn). 1875-9637/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aeolia.2013.08.003

species and moving sediment inland to maintain storm protection barriers (e.g., Arens et al., 2013a). Additionally, dune field reactivations could have global consequences via dust release (Bhattachan et al., 2012) and temperature changes (Cook et al., 2011). Future climate predictions suggest that many arid drylands will become more arid (e.g., IPCC, 2007; Seager et al., 2007) with potential implications for dune reactivation (e.g., Muhs and Maat, 1993; Wolfe, 1997; Thomas et al., 2005; Draut et al., 2012). Thus, understanding and predicting how and why dune fields transition from vegetated to active is important for mitigation, planning, and adaptation. Here, we focus on dune reactivation from blowouts under a dominantly unidirectional wind. Blowouts are landforms caused by an isolated surface disturbance in vegetation that expands through Aeolian sediment transport. In scenarios where dune fields reactivate, blowouts act as the initial sources of sediment that can subsequently advance downwind into new dunes. Blowouts are common across North America (e.g., Hugenholtz et al., 2010), South America (e.g., Zárate and Tripaldi, 2012), Europe (Provoost et al., 2011), and coastal environments worldwide (e.g., Australia: Pye, 1982). The capacity for Aeolian geomorphologists to predict dune reactivation from blowouts is limited. The root of this problem lies in a severe lack of examples – it is difficult to model and predict the behaviour without a guide. This issue has many causes. First, the process of reactivation leaves limited relict topographic evidence.

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This is in sharp contrast to stabilization, which leaves remnant dunes modified by vegetation, dune track ridges, etc., all of which can be used to help decipher stabilization processes (see Wolfe and Hugenholtz, 2009; Barchyn and Hugenholtz, 2012b,c). Second, the process of dune reactivation suggests that it could occur quicker than geomorphic-induced stabilization (which can occur on 50– 200 year timescales, Hugenholtz and Wolfe, 2005; Wolfe and Hugenholtz, 2009; Barchyn and Hugenholtz, 2012b). This timescale difference suggests that our limited remote sensing window (Hugenholtz et al., 2012) has a lower probability of recording reactivations. And finally, because dune reactivation has negative effects (Romm, 2011; Bhattachan et al., 2012; Fox et al., 2012), most arid lands are actively managed to suppress dune activity and promote vegetation growth. The few isolated studies of dune reactivation do, however, reveal some important observations. Laity (2003) described how dunes on the Mojave River (CA, USA) reactivated with a drawdown of the water table and consequent death of phreatophytic vegetation. Bate and Ferguson (1996) described how blowouts along the south coast of South Africa expanded inland due to disturbance. Redsteer et al. (2011; also Draut et al., 2012) document climate forcing in dune field reactivations in the Navajo Nation of northeast Arizona. Pye (1982) described how elongate parabolic dunes form from isolated ‘‘spot blowouts’’ in northeastern Australia. These examples are balanced by examples where blowouts have expanded and then vegetated. Blanco et al. (2008) describe how blowouts from disturbance caused by grazing cattle have expanded in southern Argentina. Fox et al. (2012) implicate bison in forming blowouts in sand hills in the Canadian prairies  100 years B.P., but the blowouts never led to widespread dune reactivation. Hugenholtz and Wolfe (2006) catalogued morphological changes in two blowouts in the Canadian prairies, but apart from morphological change, neither show evidence of reactivation. Jungerius et al. (1981) similarly documented surface changes in expanding blowouts in the Netherlands. Among these examples, perhaps the most important observation is the dichotomous behaviour. Blowouts either: (i) advance downwind and reactivate downwind sediment (leading to dune or dune field reactivation), or (ii) reach some critical size and vegetate (leading to continued dune field stability). This goal of understanding blowout behaviour has prompted many detailed airflow studies (Hugenholtz and Wolfe, 2009; Hesp and Walker, 2011; Smyth et al., 2012; Pease and Gares, 2013; among others). Most studies have found that airflow within blowouts is extremely complex and difficult to generalize, with a frustrating lack of extensible results. To introduce additional examples into the literature of dune reactivation, here we present an example of initial dune field reactivation from blowouts in the Sevier Desert of Utah, USA. The Sevier Desert has good remote sensing coverage and a diversity of reactivating dune types and behaviours. We review the dune morphology changes over the past two decades in 3 reactivation areas. Many blowouts have expanded into migrating deposition aprons. We are unable to provide a conclusive answer why these areas are reactivating, but we do discuss some possible reasons. By documenting reactivating dunes in the Sevier Desert, this study adds much needed evidence to help Aeolian geomorphologists understand how dune fields reactivate. These examples provide additional context for interpreting dune activity reconstructions based on stratigraphy and related proxies.

2. Study area The Sevier Desert is located in west-central Utah, USA, forming a portion of the Great Basin Desert. The Sevier Basin is closed;

water flows in through the Sevier River into the ephemeral Sevier Lake. Common use of the term Sevier Desert involves a larger area adjacent to the Sevier River; our focus is on the basin areas adjacent to Lynndyl, Leamington, and Delta, UT (see Fig. 1). Aeolian sediments across the desert are sourced from the postglacial Lake Bonneville (see Gilbert, 1890). Most dunes adjacent to the Sevier River are attributed to the large delta deposited into Lake Bonneville from the Sevier River while the lake was at the Provo level (Sack, 1987). Sediments from Lake Bonneville in the area are composed of glaciolacustrine and glaciodeltaic sands and silts (Oviatt, 1989). The dune fields present in the Sevier Desert are partially the result of winnowing of fines to concentrate sand particles and direct reworking of sand-sized deposits. Air photos from 1953 show most of the areas studied here to be dominantly stable, but poor resolution limits morphological analysis. Presently, the area is known for its wind erosion capacity due to regional wind field patterns, topographic wind funneling, and renowned late winter-early spring winds (Miller et al., 2012; Hahnenberger and Nicoll, 2012). Dunes in this area are actively colonized by Psoralidium lanceolatum (scurfpea), Salsola iberica (Russian thistle), and Stipa hymenoides (Indian ricegrass) (Rosenthal et al., 2005). Stabilized areas host desert shrubs Artemisia tridentate (sagebrush), and Juniperus osteosperma (juniper) (Sack, 1987). A large area of parabolic and barchan/transverse (aklé) dunes exist to the northeast of Lynndyl (UT), which are referred to as the Lynndyl Dunes by Sack (1987), or Little Sahara Dunes by Chesler et al. (1984). A portion of these dunes are within the Little Sahara National Recreation Area – a nationally important area for off-road vehicle recreation (see disturbance analyses by Dunfee, 2008). We use the term Sevier Desert Dunes to encompass the wider area across the lower Sevier River Basin. The land is used primarily for agriculture and cattle ranching and is sparsely populated (Lowe and Sanderson, 2003). While dune field reactivation negatively affects the local economy through loss of vegetated land surface, the nearby Provo-Salt Lake-Logan metropolitan area (population: >2 million) is 170 km downwind. Authorities maintain a close watch on air quality in the vicinity (see Hahnenberger and Nicoll, 2012). In July 2007, the Milford Flat wildfire burned 1460 km2 of the southern Sevier Desert (50 km southwest and upwind of study sites). Extremely high dust fluxes and poor air quality post-fire prompted local agencies to spend $25 million USD in an emergency effort to vegetatively-stabilize exposed sediment (Miller et al., 2012). Reactivation of dunes in the Sevier Desert and erosion of fresh glaciolacustrine sediment by active saltators could have similar downstream air quality consequences for the Salt Lake City metropolitan area (Hahnenberger and Nicoll, 2012). Fig. 1 shows the location of dunes within the Sevier desert. The diversity of dune activities across the Sevier Desert suggests the regional climate is near a threshold for dune stability. We have isolated three areas showing dune field reactivation within the Sevier Desert, which we have named Leamington, Champlin, and Oak City after nearby landmarks (see Fig. 1).

3. Terminology and scope Prior to discussing dune behaviour, we clarify our use of terminology. Throughout, we use the terms ‘blowout’ to describe the morphology that results when disturbance breaks through vegetation and erodes the underlying sediment. In most cases a blowout (as used here) consists of an ‘erosional hollow’ and a ‘deposition apron’. When a dune field reactivates, this deposition apron begins to migrate downwind. In some cases the deposition apron has morphology reminiscent of a parabolic dune (described below). We do not use the term ‘dune’ in this manuscript to describe migrating

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Fig. 1. Overview of dune reactivations in the Sevier Desert, UT, USA. (a) Location of the Sevier Desert within the USA. (b) Mean yearly sediment flux calculated from wind records in Delta, UT, calculated as mean sediment flux within 12 direction bins. The three reactivations discussed in this study are labelled in orange. Coordinates are UTM Zone 12N.

‘deposition aprons’ to avoid confusion – but in many cases these features have all the characteristics of a migrating dune (shallow sloped stoss, slipface, and advance). We use the term ‘reactivation’ to describe a landscape in the process of de-vegetation, encompassing both areas that have previously hosted migrating dunes, and those that have not. The underlying sediment in the Sevier Desert is difficult to map precisely – but in cases it has previously been reworked into dune forms and stabilized (particularly in the Leamington reactivation). We refer to these stabilized parabolic dune features as ‘dunes’. We focus our discussion on environments with unidirectional winds as the Sevier Desert is dominantly unidirectional. Some of what is observed in the Sevier Desert may have relevance to environments with bi-directional wind regimes and non-migrating dunes, but we refrain from speculating these links in this study.

4. Reactivation descriptions 4.1. Leamington reactivation The Leamington reactivation is the largest reactivation in the Sevier Desert. Blowouts have expanded into migrating deposition aprons, many of which have developed slipfaces (Fig. 2). The

sediment surface was predominantly vegetated in 1993 (Fig. 2a) with the exception of one semi-active remnant parabolic dune. In 2004 (Fig. 2b) blowouts appeared and expanded with unvegetated deposition aprons. By 2011 (Fig. 2d), the blowouts exposed sufficient sediment to advance downwind 30–50 m. Downwind expansion of the blowout deposition aprons appears to be a process similar to regular dune advance with an erosional stoss slope and depositional slipface. Some blowouts evacuated sufficient sediment to form a slipface, others did not. Fig. 2 shows a qualitative correlation between the formation of a slipface and the quantity of sediment that was evacuated from the blowout. In the Leamington reactivation, it is not the old dunes that are reactivating in their existing form – it is new blowouts forming on the existing dunes. For example, the parabolic dune in Fig. 2a shows no reactivation. The geomorphology of the new dunes is not a replication of the previous dune forms (see also: Blanco et al., 2008; Fox et al., 2012). Colonizer vegetation does seem to have some limited effects on the dune morphology during reactivation. In 2006 (Fig. 2c), some deposition aprons were arrested by colonizer vegetation as they expanded, but subsequently devegetated in 2011 (Fig. 2d). Some blowout aprons are beginning to resemble incipient parabolic dunes where vegetation is arresting the edges of the active patches of sand (Pye, 1982; Durán and Herrmann, 2006; Barchyn and

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may vegetate, or continue the observed trend of reactivation into the future. Second, colonizer species are not arresting these dunes, with an exception in Fig. 2c where some deposition aprons were vegetated for a short time. Third, all of the blowouts have responded with near identical timing. These observations suggest that this reactivation has a driver with a spatial scale larger than an individual blowout, and the environmental conditions are such that active dunes can be sustained in the present climate. And fourth, the area of active sand in this environment has increased from near zero in 1993 (Fig. 2a) to >50% in 2011 (Fig. 2d). This is a major change in land cover over a relatively short time. 4.2. Champlin reactivation The Champlin reactivation (Fig. 3) provides an example of sand sheet reactivation from blowouts. In 1993 (Fig. 3a) the area was vegetated. In 2004 (Fig. 3b) parabolic sand dunes appear to be partially active, but are covering a relatively small area. It is unclear here how the sediment was initially exposed, but given vegetation immediately upwind we suspect there are small blowouts caused by local disturbance that appeared between 1993 and 2004, or possibly before 1993. In Fig. 3c, more sediment is exposed and drifted downwind in sand sheets. Deposition apron front migration ranges from 300 to 600 m. Note that there is also upwind and cross-wind expansion of the area of active sand. This may be due to periodic northerly winds (see Fig. 1b), or potentially a result of upwind winnowing of a shallow blowout hollow (see Hugenholtz, 2010). In 2011 (Fig. 3d) the sediment has mobilized further and is clearing a path across roads and rangeland as it advances downwind. Note in 2011 (Fig. 3d) some sand sheets are beginning to host vegetation. It appears that this vegetation is growing on the deflation plain following the passage of a dune but the topography is low and unclear from images. 4.3. Oak City reactivation Blowouts in the Oak City reactivation (Fig. 4) provide further examples of deposition aprons advancing from isolated disturbance points as sand sheets. Contrary to the Champlin reactivation, blowouts in the Oak City reactivation were initially formed prior to 1993 (Fig. 4a). In 2004 (Fig. 4b), the blowouts expanded downwind slightly. In 2009 and 2011, (Fig. 4c and d) the blowout deposition aprons began to expand downwind much more rapidly. The deposition apron fronts migrated 150–450 m downwind. Directly upwind of the Oak City reactivation are actively migrating barchan and transverse dunes showing little signs of colonizer vegetation encroachment. Colonizer species, which showed some capacity to arrest reactivating dunes in the Leamington reactivation, appear to be absent here, or have lower vitality. Fig. 2. Dune behaviour in the Leamington reactivation (image centered on 39° 280 54.3000 N, 112° 180 36.4200 W). A black line marks a remnant parabolic dune previously stabilized as a common reference point. Red ovals mark initial blowouts that appeared between images (a) and (b) and subsequently advanced downwind in images (b), (c), and (d). Note deposition apron stabilization in (c). Image sources through Google Earth™: (a) U.S. Geological Survey, (b) DigitalGlobe, (c) State of Utah, (d) Google Earth.

Hugenholtz, 2012b,c). In some cases the original blowouts are being buried by a deposition apron advancing from upwind. This indicates the original stabilized dune field geomorphology present in 1993 (Fig. 2a) is being completely reworked. We can make four important observations about this reactivation. First, although the area of active sand increased, the deposition aprons are beginning to show a parabolic morphology and are not fully-developed barchan or transverse dunes. These dunes

5. Discussion Globally, examples of reactivations are relatively rare, and the images presented here provide valuable clues to the behaviour of dunes during reactivation. Particularly in the southwest USA, increased aridity is expected in the future (IPCC, 2007; Seager et al., 2007). As a result, there is increased potential for dune field reactivation and a resounding need for predictions. We first discuss potential causes of these reactivations before expanding on aspects of the dune behaviour. 5.1. Climate and water changes Changes in dune activity are traditionally discussed as a result of climate forcing (e.g., Lancaster, 1988; Hugenholtz and Wolfe,

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Fig. 3. Dune behaviour in the Champlin reactivation (image centered on 39° 350 09.9500 N, 112° 180 49.0200 W). Red lines mark the location outline of remnant parabolic dunes stabilized before image (a) as common reference points. Blowouts and disturbance reactivated shallow sediment which subsequently reactivated in images (b), (c) and (d). Note vegetation in (d). Image sources through Google Earth™: (a) U.S. Geological Survey, (b) DigitalGlobe, (c) United States Department of Agriculture Farm Service Agency, (d) Google Earth.

2005). Dune activity can be framed as a competition between sediment transport (promoting active sand) and vegetation growth (promoting stable, vegetation covered sand) (e.g., Durán and Herrmann, 2006). Changes in activity in response to climate are

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Fig. 4. Dune behaviour in the Oak City reactivation (image centered on 39° 170 50.1700 N, 112° 250 19.8600 W). Disturbance, initially created before image (a) expanded in images (b), (c), and (d). Note that not all blowouts expanded into sand sheets. Image sources through Google Earth™: (a) U.S. Geological Survey, (b) DigitalGlobe, (c) United States Department of Agriculture Farm Service Agency, (d) Google Earth.

generally attributable to changes in either vegetation growth or changes in sediment transport (e.g., Lancaster, 1988). To evaluate these variables in the Sevier Desert we calculated: (i) sediment flux (Q), (ii) the precipitation to potential evapotranspiration ratio (a widely used proxy for vegetation growth), and (iii) the Lancaster M (1988) dune mobility index (Fig. 5). To calculate sediment flux we used wind data from Delta, UT from 1973 to 2011. We converted wind measurements to shear velocity using the ‘Law of the Wall’, with an anemometer height

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of 10.0 m and a aerodynamic roughness length of 1/30th the grainsize diameter (mean grainsize: 0.12 mm, after Sack, 1987). We calculated a critical shear velocity of 0.196 m s1 using the Shao and Lu (2000) equation with a sediment density of 2650 kg m3 and

air density of 1.23 kg m3. We then calculated Q with the corrected White (1979) equation (Namikas and Sherman, 1997). The wind record had gaps and variable record lengths. Time missed in gaps and measurements representing greater than 6 h was replaced

Fig. 5. Climate data from Delta, UT (see Fig. 1). (a) The general status of reactivations, black lines mark images in Figs. 2–4. (b) Sediment flux (Q) shows an increasing trend, with 2010 and 2011 among the highest years. (c) Temperature, and (d) precipitation. (e) The precipitation to potential evapotranspiration ratio, dominantly within the ‘semiarid’ classification. (f) The Lancaster M dune mobility index. The grey lines mark differences between classifications ‘stable’ (0–50), ‘crests active’ (50–100), and ‘dunes active except interdunes’ (100–200).

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with an average for the year. Total sediment flux was multiplied by each record length and summed for the year, providing a yearly average sediment flux in bulk volume units (m3 m1 year1, using a bulk density of Aeolian sediment as 1500 kg m3 from Ritsema and Dekker, 1994). We removed yearly records from 1973, 1998, and 2012, which had one or more month with less than an arbitrary minimum 50 records. Mean yearly sediment flux was averaged within 12 direction bins to create Fig. 1b. Results should be considered provisional due to: (i) persistent issues with wind record quality in the Great Basin, USA (discussed by Jewell and Nicoll, 2011), (ii) our lack of treatment of winter surface conditions such as snow and ice, which increase critical shear velocity (Barchyn and Hugenholtz, 2012d), and (iii) the well acknowledged approximate nature of Aeolian sediment flux prediction equations (Sherman et al., 2012). Overall, the average yearly vector sum sediment flux was 12.40 m3 m1 year1 to an azimuth of 29.05°. Sediment flux increased slightly beyond 1993. Both 2010 and 2011 were both windy years with sediment flux almost double 2006–2009 (Fig. 5b). Jewell and Nicoll (2011) cataloged drift potential (a measure of sediment flux) across the Great Basin of the US. They found a systematic decrease in drift potential downwind at Salt Lake City (170 km downwind) from 1970 forward and increase in drift potential at Milford, UT (110 km upwind). To evaluate changes in aridity we calculated the Thornthwaite (1948) precipitation to potential evapotranspiration ratio (P/PE), which has been widely used to classify climate. Aridity is thought to correspond quite closely with vegetation vitality and the capacity for vegetation to colonize dunes. According to the P/PE record Delta, UT has a semi-arid to sub-humid climate. The years 1993 and 1994 show extremely high precipitation values, which are anomalous, but nonetheless measured. A variety of ‘dune mobility indices’ are available for relating climate to dune activity. The most widely used is the Lancaster (1988) M index (e.g., Thomas et al., 2005), which is calculated as M = W/(P/PE), where W is the percent of time the wind speed is above the critical threshold. Lancaster (1988) provided a series of classifications based on linear dune activity in southern Africa where: M < 50: stable, 50 < M < 100: crests active, 100 < M < 200: dunes active except for interdunes, and M > 200: fully active. Dunes in the Sevier Desert plot near to the boundary between stable and crests active. The years of 2000 and 2002 are of note due to regional droughts and high Q, which produced high M index values. While the local climate is likely to have some influence, it is worthwhile to consider potential changes in the adjacent Sevier River. The water in the Sevier River is completely managed for downstream irrigation through a complex series of reservoirs (http://www.sevierriver.org/; accessed 25 April 2013). Fig. 6 shows discharge from 2008 to 2013 on the Sevier River (Fig. 1). While the

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Sevier River is controlled and one measurement does not provide sufficient data to model water table depths, it does show that 2011 had much higher discharge in the Sevier River than previous years. 5.2. Causes of dune reactivation We are unable to conclusively determine the cause of reactivations in the Sevier Desert within the scope of this study, but we can discuss several possible scenarios to stimulate further research. It is attractive to implicate one factor (Laity, 2003), but realistically changes in the dunes could be due to a variety of interacting factors (Barchyn and Hugenholtz, 2013). The spatial and temporal patterns of reactivation give some clues about potential causes. Not all of the dunes within the Sevier Desert are reactivating – this suggests the cause may be spatially isolated. However, within reactivation zones, these reactivations are not composed of only one blowout. The blowouts are responding at the same time as each other (particularly in the Leamington reactivation). This suggests that a driver may be acting on the kilometer scale and it is difficult to implicate one particularly intense zone of disturbance. Changes in dune activity can be conceptually modeled as a perturbation with reaction and relaxation times (Hugenholtz and Wolfe, 2005). Identifying the activation trigger is difficult, but both 2000 and 2002 showed increases in the Lancaster M index (Fig. 5f). This is attributable both to reasonably high sediment flux (especially in 2002) and low P/PE values. The aridity could have increased the fragility of these landscapes such that a normal level of disturbance was capable of forming blowouts between 1993 and 2004 (see Barchyn and Hugenholtz, 2013). Disturbance is essential to blowout formation, but difficult to incorporate into activation models (e.g., Yizhaq et al., 2009). Furthermore, disturbance is difficult to monitor, thus making it hard to establish cause-effect linkages. At the Leamington and Champlin sites there are anthropogenic features visible in the imagery that hint at a link between the reactivations and cattle disturbance. Some of the key features indicate the presence of cattle at these sites: corrals, hay stockpiles and extensive networks of cattle trails (see Supplementary data). At Oak City there are no clear signs of cattle activity after 1993, but that does not preclude use of this area by cattle in the past. It is important to note that while cattle may have played a role in breaching the vegetation cover in order for blowouts to form, this type of disturbance is not new to this landscape. Unless stocking rates changed dramatically over the past 2 decades, it is unlikely that the reactivations are solely a response to the disturbance; rather, additional effects may be present, possibly a regional weakening of the protective vegetation skin.

Fig. 6. Discharge in the Sevier River at the stream gauge marked in Fig. 1. While the plot does show 2011 as year with high discharge, note that the Sevier River is highly controlled and this provides only a snapshot of flow conditions in the area.

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While the initial formation of blowouts begins when blowouts break through the existing vegetation, colonization of the deposition apron is possibly an important process that restrains the downwind advance and subsequent reactivation of the dune field (Hugenholtz and Wolfe, 2006; Hugenholtz et al., 2010; Barchyn and Hugenholtz, 2013). All three reactivations show little effect of colonizer species. This could be due to a regional reduction of colonizer species vitality, or potentially due to an increase in sediment flux (see Barchyn and Hugenholtz, 2012a,b,c). We may be able to implicate the sharp increase in sediment flux during 2010 and 2011 (Fig. 5b) in keeping the deposition aprons free of vegetation in 2011 images. The spatial patterning of reactivations suggests that local changes in groundwater could be worth investigating (see also Laity, 2003). Agriculture in the area is supplied by both groundwater withdrawal and a system of aqueducts fed by the Sevier River (from the mid 1800s; Gilbert, 1890). All reactivation zones are areas of groundwater discharge through evapotranspiration (Lowe and Sanderson, 2003). Measurements of the groundwater in the area show that all the reactivations considered here are in areas where surface unconfined aquifer levels have increased from 1970 to 2000 (Fig. 4, Lowe and Sanderson, 2003). However, more recent analysis is required to link with these reactivations. The large-scale management of water in the area makes precise modeling of the water table changes difficult, but Fig. 6 does provide an example of how changes in stream flow can be large from year to year. The large discharge in 2011 did not appear to result in vegetation growth in 2011 images. Of note for the Leamington reactivation is the Fool Creek #1 Reservoir (Fig. 1), which is used for ancillary water storage – we were unable to obtain historical records on levels. 5.3. Dune behaviour and morphology The reactivations described here are interesting in that they provide an example of a case where blowouts expand downwind. Many blowouts simply vegetate (Jungerius et al., 1981; Hugenholtz and Wolfe, 2006; Hugenholtz et al., 2010). It would be helpful to be able to predict this behaviour. Fundamentally blowouts are a bio-geomorphic phenomena. Both vegetation and sediment transport must be considered to effectively understand how blowouts evolve. While the cohesivity of the stabilized vegetation and soil provides some measure of how large a blowout will expand laterally (Jungerius et al., 1981; Hugenholtz and Wolfe, 2006), it is pertinent to examine the deposition apron to evaluate whether the apron will advance downwind (Barchyn and Hugenholtz, 2012a, 2013). If a deposition apron remains unvegetated, it will be more amenable for downwind advance. A lack of colonizing vegetation observed on the deposition apron implies that the environment is inhospitable. If we generalize that in these environments deposition rates are greater than the deposition tolerance of vegetation (Maun, 1998; Barchyn and Hugenholtz, 2012a), this suggests either: (i) that the colonizer species here have a low deposition tolerance (potentially low vitality), or (ii) the sediment flux and apron geometry is producing high deposition rates. Rosenthal et al. (2005) observe Psoralea lanceolatum to be among the colonizer species present within the Little Sahara dunes. P. lanceolatum is an aggressive rhyzotomous colonizer species elsewhere (e.g., Hugenholtz, 2010), suggesting that deposition aprons in reactivation zones studied may be subject to very high deposition rates. Further research is required to clarify the case in the Sevier Desert – but analysis of deposition aprons may find utility elsewhere in addressing dichotomous behaviour in blowouts, an idea shared by Barchyn and Hugenholtz (2013).

While it is impossible to time the blowout expansion perfectly, there does seem to be a relation between the depth of the blowout and advance of the deposition apron. Sand sheets (Champlin and Oak City reactivations) seemed to advance quicker across the landscape than incipient dunes with slipfaces (Leamington reactivation). This may be due to the well established inverse correlation between dune height and celerity (Bagnold, 1941). Particularly interesting is the genesis of the parabolic-like planimetric morphology of some deposition aprons (see Figs. 3 and 4). While planimetrically these appear similar to parabolic dunes – they may not form through the same mechanisms as the barchan-parabolic transformation (see Durán and Herrmann, 2006; Barchyn and Hugenholtz, 2012a). First, the shape could be due to the different timing of sediment release as the blowout expands laterally. Sediment will be released in the center of the deposition apron first, then progressively released to either side of the initial disturbance as the blowout expands, yielding a parabolic shape (Jungerius et al., 1981). Second, there could be unobservable differences in the height and volume of the deposition apron, leading to variability in advance speed (e.g., Parteli et al., 2011). 6. Summary and implications The lack of published examples of reactivating dune fields worldwide is implicitly enounced by Arens et al. (2013b): ‘‘The question here is whether stabilization [in the Netherlands] can be reversed in a sustainable way, for example, by restoring natural processes. Or will managers be forced to fight stabilization ‘‘forever’’?’’ (p. 108, Arens et al., 2013b). Of course, the notion of ‘‘fighting stabilization forever’’ is hyperbole, but the lack of reactivation examples worldwide has led to a situation where reactivation is not regularly considered. Dune fields have reactivated in the past (and will in the future). And to predict reactivations, we will require examples. The Sevier Desert of Utah, USA provides a rare glimpse of dune morphodynamics during the first stages of a dune field reactivation. Here we described three reactivation examples from blowouts revealing: (i) reactivations can be fast (years-decades), even with relatively modest sediment flux, (ii) the causes of reactivations are not always directly attributable to climate and may be due to a variety of interacting factors (some of which are difficult to measure), and (iii) the lack of colonizer vegetation on blowout deposition aprons appears to be related to dune advance. Accordingly, paleo-reconstructions of dune field reactivations based on stratigraphic evidence and other proxies require careful consideration of a suite of interacting factors affecting vegetation and sediment transport. Predicting dune field reactivation in a potentially more arid future will require more detailed modeling and further examples; dunes in the Sevier Desert could provide a base for further research.

7. Role of funding sources This study was funded by the National Science and Engineering Research Council of Canada, Alberta Innovates, and Cenovus Energy. Funders had no role in the study design, data collection, analysis, interpretation, report preparation, or the decision to submit the paper for publication. Acknowledgements The authors acknowledge financial support for this project from the National Science and Engineering Research Council, Alberta

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