Geology topics

Research about Glen Canyon National Recreation Area

Source-linked reports with geographic coverage including Glen Canyon National Recreation Area.

3 recordsLinked to original sources

USGS2019

Assessment of polycyclic aromatic hydrocarbon concentrations in southern Lake Powell, Glen Canyon National Recreation Area, Arizona and Utah, 2016–17

Polycyclic aromatic hydrocarbon contamination related to boat use is one of the most important water-quality issues affecting Lake Powell. High concentrations of polycyclic aromatic hydrocarbons in water are common around marinas and other areas with extensive motorboat activity because of releases of uncombusted or partially combusted oil and gasoline from boat engines. The fate of these compounds in Lake Powell is of serious environmental concern because of their toxicity and carcinogenicity and their moderate persistence once they enter the aquatic ecosystem. In 2016–17, the U.S. Geological Survey (USGS) assessed the presence and concentrations of polycyclic aromatic hydrocarbons in Lake Powell at seven sites where concentrations have historically been elevated and one site where concentrations have historically been relatively low. Semipermeable membrane devices were used to collect samples that represent time-weighted averages of polycyclic aromatic hydrocarbon concentrations in water over one-month deployment periods. Samples were collected from the epilimnion of the lake at each of the eight sampling sites during two periods of relatively high boat use (summer), and one period of relatively low boat use (spring). Twenty-eight out of 33 polycyclic aromatic hydrocarbons analyzed were detected in Lake Powell during the three sampling events. During the two summer sampling events, concentrations were generally higher, and more compounds were detected, than during the spring sampling event. Twenty-two of the polycyclic aromatic hydrocarbons analyzed in 2016–17 had previously been analyzed by the USGS in the summer of 2010 at the same 8 sites using the same collection method. Eleven of 22 compounds were detected in the summer 2010, summer 2016, and summer 2017 sampling events, and 1 was detected in the summer 2010 and summer 2017 samplings, but not in the summer 2016 sampling event. During both the current and previous sampling events, concentrations were generally higher, and more compounds were detected, at the high-use marina sites located most downstream on the lake. The consistent presence of a wide range of polycyclic aromatic hydrocarbons in the water of Lake Powell indicates chronic and (or) recent anthropogenic sources of contamination. The results from this study will provide the National Park Service with information necessary to determine if the current regulations on emission standards for personal watercraft used on Lake Powell are effective in lowering polycyclic aromatic hydrocarbon concentrations in the lake.

Arizona, Utah↗

USGS2014

High-resolution topography and geomorphology of select archeological sites in Glen Canyon National Recreation Area, Arizona

Along the Colorado River corridor between Glen Canyon Dam and Lees Ferry, Arizona, located some 25 km downstream from the dam, archaeological sites dating from 8,000 years before present through the modern era are located within and on top of fluvial and alluvial terraces of the prehistorically undammed river. These terraces are known to have undergone significant erosion and retreat since emplacement of Glen Canyon Dam in 1963. Land managers and policy makers associated with managing the flow of the Colorado River are interested in understanding how the operations of Glen Canyon Dam have affected the archeological sites associated with these terraces and how dam-controlled flows currently interact with other landscape-shaping processes. In 2012, the U.S. Geological Survey initiated a research project in Glen Canyon to study the types and causes of erosion of the terraces. This report provides the first step towards this understanding by presenting comparative analyses on several types of high-resolution topographic data (airborne lidar, terrestrial lidar, and airborne photogrammetry) that can be used in the future to document and analyze changes to terrace-based archaeological sites. Herein, we present topographic and geomorphologic data of four archaeological sites within a 14 km segment of Glen Canyon using each of the three data sources. In addition to comparing each method’s suitability for adequately representing the topography of the sites, we also analyze the data within each site’s context and describe the geomorphological processes responsible for erosion. Our results show that each method has its own strengths and weaknesses, and that terrestrial and airborne lidar are essentially interchangeable for many important topographic characterization and monitoring purposes. However, whereas terrestrial lidar provides enhanced capacity for feature recognition and gully morphology delineation, airborne methods (whether by way of laser or optical sensors) are better suited for reach- and regional-scale mapping. Our site-specific geomorphic analyses of the four archeological sites indicate that their current topographical conditions are a result of different and sometimes competing erosional agents, including bedrock- and terrace-based overland flow, fluvial-induced terrace bank collapse, and alluvial-fan-generated debris flows. Although the influences of anthropogenic-induced erosion from dam operations are not specifically analyzed in this report, we do identify geomorphic settings where dam operations are either more or less likely to affect archeological site stability. This information can be used to assist with future monitoring efforts of these sites and identification of similar conditions for other archeological sites along the Colorado River corridor in Glen Canyon.

Arizona↗

USGS2006

Using packrat middens to assess how grazing influences vegetation change in Glen Canyon National Recreation Area, Utah

The fossil and sub-fossil plant macrofossils and pollen grains found in packrat middens can serve as important proxies for climate and vegetation change in the arid Southwestern United States. A new application for packrat midden research is in understanding post-settlement vegetation changes caused by the grazing of domesticated animals. This work examines a series of 27 middens from Glen Canyon National Recreation Area (GLCA), spanning from 995 yr BP to the present, which detail vegetation during the periods just prior to, and following, the introduction of domesticated grazers. By comparing middens deposited before and after the start of grazing by domesticated sheep and cattle, the effect on the native plant communities through time can be determined. This analysis of change through time is augmented by measurements of change through space by contrasting contemporaneous middens from nearby similar grazed and ungrazed sites. These comparisons are only made possible by the presence of inaccessible ungrazed areas surrounded by steep cliffs. Multivariate ordinations of the plant assemblages from packrat middens demonstrated that even though all middens were selected from similar geologic substrates, soils, and vegetation type, their primary variability was site-to-site. This suggests that selecting comparable grazed versus ungrazed study treatments would be difficult, and that two similar sites several kilometers apart should not be assumed to have been the same prior to grazing without pre-grazing data. But, the changes through time on grazed areas, as well as the differences between grazed and ungrazed areas in the diversity of certain taxonomic groups, both suggest that grazing by domesticated ungulates has had a noticeable effect on the vegetation. The changes seen through time suggested that grazing lowered the number of taxa recorded and lessened the pre-existing differences within sites, homogenizing the resultant plant associations. Late Holocene pre-settlement middens, and modern middens from ungrazed areas, contained more native grasses, skunkbush sumac (Rhus trilobata), blackbrush (Coleogyne ramosissima), winterfat (Krascheninnikovia lanata), Utah serviceberry (Amelanchier utahensis), and roundleaf buffaloberry (Shepherdia rotundifolia) than modern middens from grazed areas. Pollen data supported the macrofossil data, recording decreases in pollen of the goosefoot family (Chenopodiaceae), grass family (Poaceae), and globemallow (Sphaeralcea spp.) from pre- to post-settlement.

Utah↗