Isopach Map of Postimpoundment Sediment in Lake Mead - Geographic Coordinates

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Frequently-anticipated questions:

What does this data set describe?
    Title:
    Isopach Map of Postimpoundment Sediment in Lake Mead - Geographic Coordinates
    Abstract:
    Lake Mead is a large interstate reservoir located in the Mojave Desert of southeastern Nevada and northwestern Arizona. It was impounded in 1935 by the construction of Hoover Dam and is one of a series of multi-purpose reservoirs on the Colorado River. The lake extends 183 km from the mouth of the Grand Canyon to Black Canyon, the site of Hoover Dam, and provides water for residential, commercial, industrial, recreational, and other non-agricultural users in communities across the southwestern United States. Extensive research has been conducted on Lake Mead, but a majority of the studies have involved determining levels of anthropogenic contaminants such as synthetic organic compounds, heavy metals and dissolved ions, furans/dioxins, and nutrient loading in lake water, sediment, and biota (Preissler, et al., 1998; Bevans et al, 1996; Bevans et al., 1998; Covay and Leiker, 1998; LaBounty and Horn, 1997; Paulson, 1981). By contrast, little work has focused on the sediments in the lake and the processes of deposition (Gould, 1951). To address these questions, sidescan-sonar imagery and high-resolution seismic-reflection profiles were collected throughout Lake Mead by the USGS in cooperation with researchers from University of Nevada Las Vegas (UNLV). These data allow a detailed mapping of the surficial geology and the distribution and thickness of sediment that has accumulated in the lake since the completion of Hoover Dam. Results indicate that the accumulation of post-impoundment sediment is primarily restricted to former river and stream beds that are now submerged below the lake while the margins of the lake appear to be devoid of post-impoundment sediment. The sediment cover along the original Colorado River bed is continuous and is typically greater than 10 m thick through much of its length. Sediment thickness in some areas exceeds 35 m while the smaller tributary valleys typically are filled with less than 4 m of sediment. Away from the river beds that are now covered with post-impoundment sediment, pre-impoundment alluvial deposits and rock outcrops are still exposed on the lake floor.

  1. How should this data set be cited?

    Twichell, David C. , and Cross, VeeAnn A. , 2003, Isopach Map of Postimpoundment Sediment in Lake Mead - Geographic Coordinates:.

    Online Links:

    This is part of the following larger work.

    Twichell, David C. , Cross, VeeAnn A. , and Belew, Stephen D. , 2003, Mapping the floor of Lake Mead (Nevada and Arizona): Preliminary discussion and GIS data release: Open-File Report 03-320, U.S. Geological Survey, Woods Hole Field Center, Woods Hole, MA.

  2. What geographic area does the data set cover?

    West_Bounding_Coordinate: -114.888307
    East_Bounding_Coordinate: -114.006721
    North_Bounding_Coordinate: 36.503170
    South_Bounding_Coordinate: 35.994301

  3. What does it look like?

  4. Does the data set describe conditions during a particular time period?

    Beginning_Date: 14-May-1999
    Ending_Date: 26-Apr-2001
    Currentness_Reference: ground condition

  5. What is the general form of this data set?

    Geospatial_Data_Presentation_Form: raster digital data

  6. How does the data set represent geographic features?

    1. How are geographic features stored in the data set?

      This is a Raster data set. It contains the following raster data types:

      • Dimensions 2022 x 3503 x 1, type Grid Cell

    2. What coordinate system is used to represent geographic features?

      Horizontal positions are specified in geographic coordinates, that is, latitude and longitude. Latitudes are given to the nearest 0.000252. Longitudes are given to the nearest 0.000252. Latitude and longitude values are specified in Decimal degrees.

      The horizontal datum used is North American Datum of 1983.
      The ellipsoid used is Geodetic Reference System 80.
      The semi-major axis of the ellipsoid used is 6378137.000000.
      The flattening of the ellipsoid used is 1/298.257222.

  7. How does the data set describe geographic features?

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)

  2. Who also contributed to the data set?

  3. To whom should users address questions about the data?

    David C. Twichell
    U.S. Geological Survey
    Oceanographer
    384 Woods Hole Rd.
    Woods Hole, MA 02543-1598

    (508) 548-8700 x2266 (voice)
    (508) 457-2310 (FAX)
    dtwichell@usgs.gov

Why was the data set created?
This GIS (ESRI) grid represents the integration of three datasets collected in 1999-2001 and represents the sediment thickness in Lake Mead as mapped with seismic-reflection equipment.

How was the data set created?
  1. From what previous works were the data drawn?

  2. How were the data generated, processed, and modified?

    (process 1 of 9)
    Seismic-reflecition profiles were interpreted in Landmark seismic interpretation software. The interpreted surfaces included the pre- and post-impoundment surfaces.

    (process 2 of 9)
    A supplemental horizon was created within Landmark which is the difference between the pre-and post-impoundment surfaces.

    (process 3 of 9)
    This thickness horizon was then exported from Landmark as X,Y,Z values in ASCII format.

    (process 4 of 9)
    This ASCII X,Y,Z file was then imported into Earthvision software package. Within Earthvision, these discrete points were gridded using the default spline gridding technique of earthvision.

    (process 5 of 9)
    The Earthvision grid was then exported as an X,Y,Z ASCII text file.

    (process 6 of 9)
    The Earthvision ASCII text file was then reformatted using an AWK script into an ESRI ASCII grid format.

    (process 7 of 9)
    The ESRI ASCII grid format was imported into ArcView and converted to the standard ESRI grid format.

    (process 8 of 9)
    This grid was then clipped to the sediment limit shapefile.

    (process 9 of 9)
    This UTM grid was then reprojected to the geographic coordinate system using ArcToolbox from ArcGIS version 8.2.

How reliable are the data; what problems remain in the data set?
  1. How well have the observations been checked?

  2. How accurate are the geographic locations?

    The navigation system used was a P-Code GPS system. Navigation was interpolated over as much as a 10 second interval, converted to UTM zone 11 coordinates and rounded to the nearest meter.

  3. How accurate are the heights or depths?

    The nominal resolution of the chirp subbottom system is assumed to be 0.5 meters. Another uncertainty is the speed of sound in sediment, which for this study is assumed to be 1500 m/sec. Due to the generally thin nature of the sediment cover, inaccuracies in the speed of sound are not thought to have a high impact.

  4. Where are the gaps in the data? What is missing?

    This dataset is complete for the time period it covers.

  5. How consistent are the relationships among the observations, including topology?

    This isopach map represents the integration of three years of data collection. This time frame of data collection can introduce small discrepancies in the sediment thickness.

How can someone get a copy of the data set?
Are there legal restrictions on access or use of the data?

Access_Constraints: NONE
Use_Constraints:
The U.S. Geological Survey must be referenced as the originator of the dataset in any future products or research derived from these data.

  1. Who distributes the data set? (Distributor 1 of 1)

    David C. Twichell
    U.S. Geological Survey
    Oceanographer
    384 Woods Hole Rd.
    Woods Hole, MA 02543-1598

    (508) 548-8700 x2266 (voice)
    (508) 457-2310 (FAX)
    dtwichell@usgs.gov

  2. What's the catalog number I need to order this data set?

    Downloadable Data

  3. What legal disclaimers am I supposed to read?

    These data were prepared by an agency of the United States Government. Neither the United States government nor any agency thereof, nor any of their employees, make any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed in this report, or represents that its use would not infringe privately owned rights. Reference therein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. Any views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Although all data published of this report have been used by the USGS, no warranty, expressed or implied, is made by the USGS as to the accuracy of the data and related materials and/or the functioning of the software. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of this data, software, or related materials.

  4. How can I download or order the data?

Who wrote the metadata?
Dates:
Last modified: 06-Oct-2003

Metadata author:
VeeAnn A. Cross
U.S. Geological Survey
Marine Geologist
384 Woods Hole Rd.
Woods Hole, MA 02543-1598

(508) 548-8700 x2251 (voice)
(508) 457-2310 (FAX)
vatnipp@usgs.gov

Metadata standard:
FGDC Content Standards for Digital Geospatial Metadata (FGDC-STD-001-1998)

Metadata extensions used:

Generated by mp version 2.7.3 on Mon Oct 06 12:10:03 2003