Glen Canyon National Recreation Area Research Articles

The John Wesley Powell Fossil Track Block—theropod tracks with ornithopod-like morphology from the Early Jurassic Navajo Sandstone, Glen Canyon National Recreation Area, Utah-Arizona

A large fallen block of Early Jurassic Navajo Sandstone located at Lake Powell, within Glen Canyon National Recreation Area, south-central Utah, displays natural casts of vertebrate tracks. The footprints occur on at least three track-bearing horizons preserved on and between stromatolitic sandstone beds. Two large, parallel trackways, plus a third, divergent trackway, on the main track layer (MTL) superficially resemble ornithopod footprints; however, they were produced by large-sized theropod dinosaurs, ratherthan ornithischians, and we identify these as Eubrontes. Small coelophysoid theropod tracks (Grallator) are the most common vertebrate ichnofossils on all track-bearing horizons, with approximately 50 footprints preserved on the MTL, six on the highest surface, and three on thinner float slabs stratigraphically lower in section. An additional 12 tracks in three trackways of Anchisauripus size occur on the MTL, but they superficially resemble Kayentapus in having wider divarication angles than typical Anchisauripus. The MTL also preserves at least five closely associated tetradactyl footprints that we identify as cf. Brasilichnium. A nearby, smaller fallen block preserves distinct Batrachopus tracks, which are rare in eolian environments. The microbial (possibly endoevaporitic) mats and stromatolitic horizons on which the animals had walked produced a distinct ichnomorphologic variation because of substrate consistency and the elastic properties of the mats, resulting in differential compaction of the bedding surfaces. Lithic compaction of the finer-grained sediments between denser, more resistant sandstone beds pre- and/or post-lithification resulted in additional deformation of the tracks, followed by natural erosion. We interpret these natural cast footprints on the MTL as possible transmitted tracks. The track-bearing, microbial-mat surfaces represent interdunal pooling of water, probably during periods of increased precipitation and/or rising water tables during wet seasons.

A Glimpse Through Time: Preliminary Report of the Andre’s Alcove Tracksite, Lower Jurassic Navajo Sandstone, Glen Canyon National Recreation Area, Utah

In November 2018, a team of researchers from the National Park Service and the St. George Dinosaur Discovery Site documented fossil vertebrate tracks at the Andre’s Alcove Tracksite (AAT) in the Lower Jurassic (?190–185 Ma) Navajo Sandstone in Glen Canyon National Recreation Area, Utah. The tracks at the AAT were photo-documented for photogrammetric processing using Agisoft Photoscan Professional to enable the track surface to be studied remotely. Ichnotaxa already identified at the AAT include those of small ornithischian dinosaurs (Anomoepus), theropod dinosaurs (Grallator, Eubrontes), and protosuchian crocodylomorphs (Batrachopus). Vertebrate body fossils are rarer in the Navajo Sandstone than in underlying, earlier Jurassic strata; the AAT site therefore provides important insights into late Early Jurassic vertebrate diversity and behaviors at the onset of the Navajo Erg in the American Southwest.

Integrative Biodiversity Inventories: Characterizing Lichen-Forming Fungal Diversity in Glen Canyon National Recreation Area Using DNA Barcoding and Vouchered Specimens

El río Colorado y sus afluentes en la meseta de Colorado albergan ecosistemas fluviales desérticos únicos y condiciones ambientales cambiantes. Dentro de esta región, el Área Recreativa Nacional de Glen Canyon (GCNRA, por sus siglas en inglés) se compone de un terreno desértico irregular y elevado, el cual es administrado por el Servicio de Parques Nacionales de los Estados Unidos como área recreativa y de conservación. A pesar de la importancia ecológica y económica de GCNRA, varios componentes importantes de las comunidades ecológicas que allí se encuentran permanecen mal caracterizados, incluidos los líquenes. El caracterizar la diversidad precisa de los hongos formadores de líquenes representa un desafío debido a que se trata de grupos taxonómicos poco conocidos, regiones/hábitats escasamente explorados y diversas interpretaciones de las diferencias morfológicas, incluido el reconocimiento de formas modificadas por el medio ambiente. Para comprender mejor la diversidad de líquenes en GCNRA, utilizamos un enfoque taxonómico integrador, incorporando tanto la identificación tradicional basada en la morfología como la información del marcador de código de barras de ADN de hongos estándar (marcador ITS), para compilar un inventario completo de hongos formadores de líquenes en Fifty-Mile Canyon. En 2019 se recolectaron especímenes de liquen, y de estos se secuenció el marcador ITS. Los linajes candidatos a nivel de especie se delimitaron a partir de alineaciones de secuencias múltiples a nivel de familia, utilizando el servidor web Assemble Species by Automatic Partitioning. Posteriormente, se evaluó los especímenes que comprenden especies candidatas basadas en el ADN, utilizando cuando fue posible, caracteres tradicionales de diagnóstico taxonómico para vincularlos con las especies descritas actualmente. En Fifty-Mile Canyon, documentamos 100 especies putativas de 15 familias, cada una representada por especímenes certificados, datos de secuencia de ITS y documentación fotográfica. A modo de comparación, un muestreo de registros históricos de GCNRA reveló un total de 124 especies de hongos formadores de líquenes, documentadas en toda la NRA y las tierras adyacentes. Aproximadamente el 50% de las especies documentadas en Fifty-Mile Canyon no se habían encontrado previamente en GCNRA, y proporciones similares de diversidad de especies se han encontrado en GCNRA, pero no se observaron en nuestro estudio. Registramos tres especies nuevas en Norteamérica: Calogaya ferrugineoides (H. Magn.) Arup, Frödén & Søchting, Endocarpon deserticola T. Zhang, X.L. Wei & J.C. Wei y Xanthocarpia ferrarii (Bagl.) Frödén, Arup & Søchting, verificadas usando datos de secuencia de ITS. Además, describimos formalmente a Circinaria squamulosa sp. nov., actualmente sólo conocido en losas de arenisca en Fifty-Mile Canyon. Sin embargo, la identidad taxonómica de muchas de las especies candidatas de Fifty-Mile Canyon permaneció ambigua a nivel de especie, y algunas colecciones probablemente representan linajes a nivel de especie no descritos. Nuestros resultados revelaron una gran e inesperada diversidad a nivel de especie de hongos formadores de líquenes a escala local y la diversidad general de líquenes en todo el GCNRA probablemente esté muy subestimada. Estos datos, incluidos los códigos de barras de ADN de la gran mayoría de los hongos formadores de líquenes que se encuentran en este cañón, proporcionan un recurso importante que puede integrarse en investigaciones posteriores acerca de la biodiversidad de líquenes en el suroeste de los Estados Unidos y otros climas semiáridos.

Effects of Reservoir Levels on Arizona National Recreation Area Visitation, Visitor Spending, and Local Economies

AbstractThis study estimates the effects of reservoir levels on demand for recreation visits to Lake Powell (Glen Canyon National Recreation Area [NRA]) and Lake Mead NRA, correlating reservoir levels with overnight and total recreation visits. We also consider the effects of closures of recreation access points (such as launch ramps or marinas) when lake levels fall below critical thresholds. Our overall results are similar to past studies, but find that the access point closures are a more robust predictor of visits than simple elevation measures. Policies that manage the Colorado River, including those that seek to maintain reservoir levels above critical levels with the primary goal of preventing water delivery cutbacks, also affect recreation demand and the economies of nearby communities. Our analysis maps changes in visits (from changing elevations) to changes in visitor spending, recreationist user benefits, and regional economic indicators such as value added and employment. Such indicators could be used in future benefit‐cost or economic impact analyses of Colorado River water management policies.

Bats (Chiroptera) of Glen Canyon National Recreation Area, Utah and Arizona

Glen Canyon National Recreation Area covers large and geographically unique regions of southern Utah and northern Arizona. However, multiple surveys of bats from 1952 to 2017 have not been published. Prior to 2015, most bat surveys in the recreation area were conducted using mist nets only and did not include acoustic monitoring. In this paper we compile records of bats found in Glen Canyon National Recreation Area from 1952 to 2017, including records from our recent surveys using both mist nets and acoustic sampling. During our recent surveys, 2 new species were captured (Nyctinomops macrotis and Lasionycteris noctivagans), and 2 other new species were documented using acoustic detection (Myotis evotis and Eumops perotis), bringing the total documented bat species in Glen Canyon National Recreation Area to 17 species. This paper sheds light on the diverse bat assemblage of Glen Canyon National Recreation Area that will serve as a baseline for future research and conservation efforts.

Prioritizing Archaeological Inventory and Protection with Predictive Probability Models at Glen Canyon National Recreation Area, USA

To prioritize archaeological site inventory and protection at Glen Canyon National Recreation Area (Glen Canyon NRA), predictive probability models were developed using the Random Forests machine learning method. Glen Canyon NRA consists of approximately 5,000 km2 in Arizona and Utah containing evidence of multiple prehistoric cultures spanning at least 10,000 years. Large portions of Glen Canyon NRA have never been inventoried for archaeological resources. Archaeological sites are potentially subject to irreparable damage along roads and receding Lake Powell shorelines accessible to off-road vehicles and boats. The diverse cultural history, highly variable physiographic environments, archaeological site data inconsistencies, and limited available model variable data provided challenges. Model results improved by classifying sites, based on their inferred usage, and distinguishing distinct physiographic regions within the larger NRA landscape. Model results are being used to target ongoing field surveys. Initial validation is positive, with new sites discovered in areas of predicted high probability.

Gas bubble cavities in deltaic muds, Lake Powell delta, Glen Canyon National Recreation Area, Hite, Utah

Abstract Cavities produced by migrating gas, mainly methane, emanating from the near subsurface are a rarely reported sedimentary structure from either modern or ancient clastic sediments. The delta of the Colorado River in Lake Powell, Glen Canyon National Recreation Area provides an excellent recently accessible locality for examining soft sediment deformation structures generated by escaping gases and fluids. Sub-meter-scale to tens-of-meter-scale structures, including craters, salses, and sand and mud volcanoes are present on the modern Lake Powell delta. Abundant sub-centimeter-scale cavities are found in excavations through crater sediments and exposed along cut-bank exposures of the Colorado River. In cross section the cavities are subvertical, irregular-to sigmoidal-to lenticular-shaped, 1–3 mm short axes, up to ∼3.0 cm long axes, and ∼1 cm intermediate axes. Cavities are developed in cm-scale, laminated silty clay and clay graded beds. The tops of the cavities are wider than their bases. The cavities observed in the Lake Powell delta are interpreted as mode 1 fractures and probably represent a clastic morphologic analog to ‘molar-tooth’ carbonate structures. The characteristics of the cavities, and the geometries and deformation of the surrounding delta-mud host, are consistent with bubbles and fracturing generated by gas migration experiments and theoretical calculations but vary because of inherent heterogeneity of the sediments. Fractures developed as gas migrated into pre-existing cavities causing overpressure in cavities and leading to subvertical mode 1 fracture propagation. Recent sudden decompression related to rapid lake level drops of Lake Powell may have enhanced the release of gas, primarily bacterially produced, leading to the development of these soft sediment deformation structures.

Remote sensing of tamarisk beetle (Diorhabda carinulata) impacts along 412 km of the Colorado River in the Grand Canyon, Arizona, USA

Tamarisk (Tamarix spp.) is an invasive plant species that is rapidly expanding along arid and semi-arid rivers in the western United States. A biocontrol agent, tamarisk beetle (Diorhabda carinulata), was released in 2001 in California, Colorado, Utah, and Texas. In 2009, the tamarisk beetle was found further south than anticipated in the Colorado River ecosystem within the Grand Canyon National Park and Glen Canyon National Recreation Area. Our objectives were to classify tamarisk stands along 412 km of the Colorado River from the Glen Canyon Dam through the Grand Canyon National Park using 2009 aerial, high spatial resolution multispectral imagery, and then quantify tamarisk beetle impacts by comparing the pre-beetle images from 2009 with 2013 post-beetle images. We classified tamarisk presence in 2009 using the Mahalanobis Distance method with a total of 2500 training samples, and assessed the classification accuracy with an independent set of 7858 samples across 49 image quads. A total of 214 ha of tamarisk were detected in 2009 along the Colorado River, where each image quad, on average, included an 8.4 km segment of the river. Tamarisk detection accuracies varied across the 49 image quads, but the combined overall accuracy across the entire study region was 74%. Using the Normalized Difference Vegetation Index (NDVI) from 2009 and 2013 with a region-specific ratio of >1.5 decline between the two image dates (2009NDVI/2013NDVI), we detected tamarisk defoliation due to beetle herbivory. The total beetle-impacted tamarisk area was 32 ha across the study region, where tamarisk defoliation ranged 1–86% at the local levels. Our tamarisk classification can aid long-term efforts to monitor the spread and impact of the beetle along the river and the eventual mortality of tamarisk due to beetle impacts. Identifying areas of tamarisk defoliation is a useful ecological indicator for managers to plan restoration and tamarisk removal efforts.

Identifying cost-effective invasive species control to enhance endangered species populations in the Grand Canyon, USA

Recovering endangered species populations when confronted with the threat of invasive species is an ongoing natural resource management challenge. While eradication of the invasive species is often the optimal economic solution, it may not be a feasible nor desirable management action in other cases. For example, when invasive species are desired in one area, but disperse into areas managed for endangered species, managers may be interested in persistent, but cost-effective means of managing dispersers rather than eradicating the source. In the Colorado River, a nonnative rainbow trout (Oncorhynchus mykiss) sport fishery is desired within Glen Canyon National Recreation Area, however, dispersal downriver into the Grand Canyon National Park is not desired as rainbow trout negatively affect endangered humpback chub (Gila cypha). Here, we developed a bioeconomic model incorporating population abundance goals and cost-effectiveness analyses to approximate the optimal control strategies for invasive rainbow trout conditional on achieving endangered humpback chub adult population abundance goals. Model results indicated that the most cost-effective approach to achieve target adult humpback chub abundance was a high level of rainbow trout control over moderately high rainbow trout population abundance. Adult humpback chub abundance goals were achieved at relatively low rainbow trout abundance and control measures were not cost-effective at relatively high rainbow trout abundance. Our model considered population level dynamics, species interaction and economic costs in a multi-objective decision framework to provide a preferred solution to long-run management of invasive and native species.

Remote Sensing of Tamarisk Biomass, Insect Herbivory, and Defoliation: Novel Methods in the Grand Canyon Region, Arizona

Tamarisk is an invasive, riparian shrub species in the southwestern USA. The northern tamarisk beetle (Diorhabda carinulata) has been introduced to several states to control tamarisk. We classified tamarisk distribution in the Glen Canyon National Recreation Area, Arizona using 0.2m resolution, airborne multispectral data and estimated tamarisk beetle effects (overall accuracy of 86 percent) leading to leaf defoliation in a 49,408m2 area. We also estimated individual tamarisk tree biomass and their uncertainties using airborne lidar data (100points/m2). On average, total aboveground tamarisk biomass was 8.68kg/m2 (SD = 17.6). The tamarisk beetle defoliation resulted in a mean leaf biomass loss of 0.52kg/m2 and an equivalent of 25,692kg across the entire study area. Our defoliated tamarisk map and biomass estimates can help inform restoration treatments to reduce tamarisk. Continued monitoring of tamarisk and tamarisk beetle effects are recommended to understand the currently-unknown eventual equilibrium between the two species and the cascading effects on ecosystem processes.

Economic Value of Angling on the Colorado River at Lees Ferry: Using Secondary Data to Estimate the Influence of Seasonality

AbstractGlen Canyon Dam (GCD) on the Colorado River in northern Arizona provides water storage, flood control, and power system benefits to approximately 40 million people who rely on water and energy resources in the Colorado River basin. Downstream resources (e.g., angling, whitewater floating) in Glen Canyon National Recreation Area (GCNRA) and Grand Canyon National Park are impacted by the operation of GCD. The GCD Adaptive Management Program was established in 1997 to monitor and research the effects of dam operations on the downstream environment. We utilized secondary survey data and an individual observation travel cost model to estimate the net economic benefit of angling in GCNRA for each season and each type of angler. As expected, the demand for angling decreased with increasing travel cost; the annual value of angling at Lees Ferry totaled US$2.7 million at 2014 visitation levels. Demand for angling was also affected by season, with per‐trip values of $210 in the summer, $237 in the spring, $261 in the fall, and $399 in the winter. This information provides insight into the ways in which anglers are potentially impacted by seasonal GCD operations and adaptive management experiments aimed at improving downstream resource conditions.Received October 7, 2015; accepted June 13, 2016 Published online September 30, 2016

Remote Sensing of Tamarisk Biomass, Insect Herbivory, and Defoliation: Novel Methods in the Grand Canyon Region, Arizona

Tamarisk is an invasive, riparian shrub species in the southwestern USA. The northern tamarisk beetle (Diorhabda carinulata) has been introduced to several states to control tamarisk. We classified tamarisk distribution in the Glen Canyon National Recreation Area, Arizona using 0.2 m resolution, airborne multispectral data and estimated tamarisk beetle effects (overall accuracy of 86 percent) leading to leaf defoliation in a 49,408 m2 area. We also estimated individual tamarisk tree biomass and their uncertainties using airborne lidar data (100 points/m2). On average, total aboveground tamarisk biomass was 8.68 kg/m2 (<small>SD</small> = 17.6). The tamarisk beetle defoliation resulted in a mean leaf biomass loss of 0.52 kg/m2 and an equivalent of 25,692 kg across the entire study area. Our defoliated tamarisk map and biomass estimates can help inform restoration treatments to reduce tamarisk. Continued monitoring of tamarisk and tamarisk beetle effects are recommended to understand the currently-unknown eventual equilibrium between the two species and the cascading effects on ecosystem processes.

Subsurface structure of water–gas escape features revealed by ground-penetrating radar and electrical resistivity tomography, Glen Canyon National Recreation Area, Lake Powell delta, Utah, USA

Data gathered by electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) were used to produce a three-dimensional image of subsurface soft-sediment deformation structures developed on the modern Lake Powell delta at Hite, Utah. ERT and GPR lines were run orthogonal across the crater. ERT resolved a low-resistivity layer up to 2m thick in the area near the vents within the crater. This low-resistivity layer thins toward the margins representing clays ejected from the vents. Below and adjacent to this layer is a high-resistivity layer that reflects delta top and pro-delta sands. The deepest zone recognized in the ERT profiles consists of a low-resistivity layer, clay deposits that accumulated during the maximum lake high stand. This clay zone is connected to the vent within the crater by a conduit that changes diameter vertically. GPR profiles recognized the presence of collapse features restricted to the proximity of the vent. The geometry of the model is consistent with those proposed for marine pockmarks that can be generated seismically or aseismically with the exception of subaerial exposure after the dome stage development.

Hydrocarbon-induced diagenetic alteration of the Permian White Rim Sandstone, Elaterite Basin, southeast Utah

The Permian White Rim Sandstone is a partly exhumed, primary reservoir of the Tar Sand Triangle accumulation in southeastern Utah. In the Elaterite Basin (Canyonlands National Park and Glen Canyon National Recreation Area), the White Rim Sandstone is well exposed and varies in color from white to red, orange, and brown. Superimposed on eolian and marine deposits are three diagenetic facies that impart the coloration and are defined by iron oxide cement concentration as (1) bleached white (low iron), (2) diffuse (moderate iron), and (3) concretionary (concentrated iron). A yellow alteration aureole of bleaching extends up to 10 m (32 ft) into the underlying Organ Rock Shale and up to 20 m (65 ft) into the overlying Moenkopi Formation. These formations surround the White Rim reservoir as fine-grained seals. Field, petrographic, and geochemical analyses indicate that the White Rim Sandstone records three major diagenetic stages. (1) The reservoir underwent oxidation, which led to the precipitation of thin iron grain coatings. (2) Hydrocarbon migration through the reservoir removed early grain coatings and reprecipitated disseminated and concentrated pyrite cement. (3) The pyrite was later altered to hematite or goethite by oxidizing fluids. In conventional petroleum exploration, the timing of hydrocarbon migration is often difficult to resolve. This study utilizes the record of mobilized and reprecipitated iron as a tool to constrain interpretations of the timing of hydrocarbon migration relative to seal and trap emplacement. This study has broad application as an exploration tool for deciphering fluid flow in similar clastic reservoirs.

Assessing Early Spanish Explorer Routes Through Authentication of Rock Inscriptions*

Rock inscriptions containing both names and calendar dates provide place-specific data on travels of explorers, if those inscriptions are truly authentic. We exemplify here a new strategy for determining the authenticity of inscriptions in arid environments in two case studies. One is an inscription purportedly created during the Marcos de Niza expedition of 1539 through Arizona. The other might have been made by the Dominguez–Escalante expedition of 1776 through the Colorado Plateau and Great Basin. The rock inscription in Phoenix, Arizona, “Fr Marcos de Niza corona todo el nuebo Mexico a su costa ano de 1539,” is likely not authentic. Although the Marcos de Niza petroglyph was manufactured before the use of leaded gasoline about 1922, it was made after the Little Ice Age ended in the mid-nineteenth century. In contrast, the engraving “Paso Por Aqui—Año 1776” near Lake Powell's Padre Bay in Glen Canyon National Recreation Area has a lead profile indicating that the engraving predates twentieth-century pollution and also contains a Little Ice Age signal, evidence that the engraving is likely authentic. Nearby graffiti and natural weathering often endangers rock inscriptions, necessitating conservation efforts of authentic engravings. Conservation efforts to protect the delicate condition of the Lake Powell engraving are justified by these findings. In contrast, unnecessary expenditures and effort can result from work on engravings that are not authentic.

Birds of Glen Canyon National Recreation Area, Utah and Arizona

The birds of Glen Canyon National Recreation Area, in southern Utah and northern Arizona, are documented, with information on abundance, distribution, ecology, residency status, and breeding status. In all, 316 species have been reported from the park since the first surveys in the region during the 1930s. Historical survey data from the pre-Glen Canyon Dam surveys are incorporated, as well as significant work done in the park since 1992. Breeding is documented for 87 species, including 82 native species and 5 exotic species. Many rare species are reported for the first time from this section of the state, and several recent range extensions are documented.

Unusual occurrence of gastroliths in a polycotylid plesiosaur from the Upper Cretaceous Tropic Shale, southern Utah

A polycotylid plesiosaur (Dolichorhynchops sp.), recently discovered in the Upper Cretaceous (Cenomanian-Turonian) Tropic Shale in Glen Canyon National Recreation Area, southern Utah, is associated with 289 gastroliths. This specimen is significant due to the general lack of gastroliths associated with most short-necked plesiosaur skeletons. The skeleton (MNA V10046) was excavated from a stratigraphic interval of marine shale that does not contain coarser-grained material. The stones were generally concentrated in one area of the skeleton, with the majority situated near the back of the skull. The stones are mainly composed of dark grey chert, are smooth and well rounded, and have varying degrees of polish. The majority of the stones are spherical in shape and are likely fluvial in origin. The gastroliths from this animal are similar in shape and number to those documented from elasmosaurid plesiosaurs. The gastroliths from MNA V10046, however, are much smaller and weigh considerably less than most elasmosaurid stomach stones. Given the lack of comparative material from other short-necked plesiosaurs, the size and mass differences are attributed to differences in function of gastroliths between long- and short-necked plesiosaurs.

Search and Rescue Trends and the Emergency Medical Service Workload in Utah's National Parks

To identify the emergency medical service (EMS) workload and trends associated with search and rescue (SAR) operations in Utah's National Park Service (NPS) units. Data for this study were collected from the Annual Emergency Medical Services Report and the Annual Search and Rescue Report for National Park Service units in Utah from 2001-2005. There were 4762 EMS incidents reported between 2001 and 2005, including 79 fatalities (50 traumatic; 29 nontraumatic). The most common EMS transportation method was ground (1505) and helicopter (553) transport. The heaviest trauma, medical, and cardiac workload was at Glen Canyon National Recreation Area (GLCA) and the heaviest first aid workload was at Zion National Park (ZION). There were 1190 SAR operations between 2001 and 2005 involving 67 fatalities, 623 ill or injured visitors, 1813 non-ill or non-injured visitors, and 92 saves. GLCA and ZION accounted for 47% and 21% of all SAR operations. The total cost of SAR operations was $1 363 920. SAR operations most commonly occurred on weekends, involved male visitors (59%), visitors aged 20-29 years (23%), and 40-49 years (20%), and visitors participating in day hiking (221), motorized boating (196), and canyoneering (98) activities. Most SAR operations were in lake (226), desert (147), and canyon (140) environments and were resolved within 24 hours. GLCA and ZION experienced heavy use of EMS resources that should be noted by EMS administrators and planners. GLCA and ZION also reported the most SAR operations. The development of techniques to prevent the need for SAR at GLCA and ZION would likely have the most potential to reduce the financial impact of SAR incidents and morbidity and mortality to visitors.

Fire History of Piñon-juniper Woodlands on Navajo Point, Glen Canyon National Recreation Area

Navajo Point, on the southeast tip of the Kaiparowits Plateau, supports Pinus edullis Engelm. var edulis-Juniperus osteosperma (Torrey) Little (piñon-juniper) woodlands undisturbed by large wildfires in the recent past. We developed a fire history and characterized the current fuel structure and plant biodiversity in the piñon-juniper woodlands on Navajo Point. Using a combination of 18 stand ages, stand structural characteristics classified from satellite imagery, and line intercept sampling, we determined that it would take 400–600 years to burn a cumulative area equal in extent to Navajo Point. Despite a long history of livestock grazing and fire suppression policies, the woodlands on Navajo Point still retain most of their primeval character. Specifically, the landscape patch mosaic on Navajo Point has not been fundamentally altered by 20th century fire exclusion. We conclude that the old-growth woodlands that cover at least half of Navajo Point are a natural and ecologically significant component of this ecosystem, resulting from the combination of the area's soils, climate, and inherently infrequent disturbance regime. Today, the rich understory supports 207 plant species of which only 6% are exotic. Despite the current pristine character of this woodland, the greatest potential threat comes from Bromus tectorum L. (cheatgrass), which occurs intermittently throughout the area. If expansion occurs, this flashy fuel (which has no native counterpart) could significantly shorten the centuries-long fire cycle that has allowed for the development of the biologically rich woodland on Navajo Point.

Conical sandstone landforms cored with clastic pipes in Glen Canyon National Recreation Area, southeastern Utah

Clusters of conical sandstone landforms, many with summit weathering pits, have developed on barren outcrops of the Jurassic Entrada Sandstone in Glen Canyon National Recreation Area, southeastern Utah. The conical landforms have developed on cylindrical bodies of fluidized sandstone (clastic pipes) that typically have near-vertical contacts with the enclosing cross-bedded, eolian sandstone. These landforms vary in size and shape due chiefly to differential erosion of the clastic pipe relative to the enclosing sandstone. The greater resistance to weathering of the clastic pipes is due in part to their higher content of calcite cement. Conical, pipe-cored landforms develop progressively from low domes to cones as high as 70 m. Some of the clastic pipes have relatively soft cores and resistant contacts, leading to the development of conical landforms with summit weathering pits. With time, the size of these pits increases as does the relief of the conical landform. The summit pits are as deep as 16 m and have width–depth ratios as low as 1.5. The resistant rims of these pits are due in part to calcite-enriched pipe contacts. Sandy pit-floor sediment is removed principally by strong wind rotors and vortices. Intense eolian activity in and near the landforms is indicated by abrasional features and pit-floor sand dunes. Factors that promote the development of these conical landforms include (i) the presence of clastic pipes, some with relatively soft cores; (ii) porous, friable, fine-grained pipe and host sandstones; (iii) aridity; (iv) strong winds; and (v) virtually sediment-free, unvegetated bedrock outcrops.