Interpretation of the Sidescan-sonar Imagery in Lake Mohave

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• What does this data set describe?
  1. How should this data set be cited?
  2. What geographic area does the data set cover?
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
  5. What is the general form of this data set?
  6. How does the data set represent geographic features?
  7. How does the data set describe geographic features?
• Who produced the data set?
  1. Who are the originators of the data set?
  2. Who also contributed to the data set?
  3. To whom should users address questions about the data?
• Why was the data set created?
• How was the data set created?
  1. From what previous works were the data drawn?
  2. How were the data generated, processed, and modified?
• 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?
  3. How accurate are the heights or depths?
  4. Where are the gaps in the data? What is missing?
  5. How consistent are the relationships among the data, including topology?
• How can someone get a copy of the data set?
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Title: Interpretation of the Sidescan-sonar Imagery in Lake Mohave

Abstract:

Lake Mohave is one of several multi-purpose reservoirs that have been constructed on the Colorado River. The lake was formed upon completion of the Davis Dam in 1953. No mapping of the floor of the lake had been conducted since completion of the Davis Dam. The U.S. Geological Survey, in cooperation with researchers from the University of Nevada Las Vegas, completed a geophysical survey of this lake in April 2002. The survey included collection of sidescan sonar imagery of nearly the entire lake floor, and high-resolution seismic-reflection profiles along widely spaced lines throughout the lake. The detailed mapping of the lake floor was used to determine the amount of sediment that had accumulated in the lake since impoundment, its distribution, and the processes of deposition.

1. How should this data set be cited?

   Twichell, David C. , and Foster, David S. , 2005, Interpretation of the Sidescan-sonar Imagery in Lake Mohave:.

   Online Links:

   • <http://pubs.usgs.gov/of/2004/1247/data/interpretation/sscan_interp.zip>

   This is part of the following larger work.

   Cross, VeeAnn A. , Foster, David S. , and Twichell, David C. , 2005, Lake Mohave Geophysical Survey 2002: GIS Data Release: Open-File Report 2004-1247, U.S. Geological Survey, Woods Hole Science Center, Woods Hole, MA.

   Online Links:

   • <http://pubs.usgs.gov/of/2004/1247>

2. What geographic area does the data set cover?

   West_Bounding_Coordinate: -114.713653

   East_Bounding_Coordinate: -114.570320

   North_Bounding_Coordinate: 35.869129

   South_Bounding_Coordinate: 35.200015

3. What does it look like?

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

   Beginning_Date: 18-Apr-2002

   Ending_Date: 25-Apr-2002

   Currentness_Reference: ground condition

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

   Geospatial_Data_Presentation_Form: vector digital data

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

      This is a Vector data set. It contains the following vector data types (SDTS terminology):

      • G-polygon (584)

   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 0.00001. Longitudes are given to the nearest 0.00001. 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?

   sscan_interp

   FID

   Internal feature number. (Source: ESRI)

   Sequential unique whole numbers that are automatically generated.

   Shape

   Feature geometry. (Source: ESRI)

   Coordinates defining the features.

   ID

   Number automatically generated when each polygon feature was created. (Source: Software generated.)

   Value	Definition
   0

   INTERP

   Abbreviation indicating the interpretation of the sidescan-sonar imagery. (Source: David C. Twichell)

   character set

   AREA

   Area of the polygon, in square meters, based on the UTM, Zone11, NAD83 projection. (Source: Software calculated.)

   Range of values
   Minimum:	1033.386
   Maximum:	16677829.418
   Units:	square meters

   PERIMETER

   Perimeter of the polygon, in meters, based on the UTM, Zone11, NAD83 projection. (Source: Software calculated.)

   Range of values
   Minimum:	137.794
   Maximum:	45497.027
   Units:	meters

   Entity_and_Attribute_Overview:

   What follows is a more detailed description of the INTERP field in the shapefile. In this study, in referring to sidescan-sonar data, high-backscatter is highly reflective data with pixel values towards white, or a value of 255. Low-backscatter indicated low pixel values towards black, or values of 0.

   Alluvial: Pre-impoundment alluvial fan deposits. The interpretation is based both on the pre-impoundment topography and the sidescan-sonar imagery. The imagery commonly shows a fine network of channels on the fan surfaces.

   rw_alluvial: reworked alluvial fan deposits. These deposits are found in the former Colorado River channel off of alluvial fans. Usually there is a scarp/bank that separates the alluvial fan from the alluvium that has been reworked by the former river. These appear to be pre-impoundment deposits.

   overbank: pre-impoundment overbank deposits occupy low-lying flat areas to either side of the pre-impoundment Colorado River channel. These areas usually are low-backscatter on the sidescan imagery often with high-backscatter spots. The interpretation draws both on the sidescan imagery and the pre-dam topography. The topography shows that these low-lying areas often were populated with cottonwood trees. The white spots on the sidescan image may be drowned trees, and the low-backscatter nature of the return may be because the sediment is finger-grained.

   dunes: sub-aerial sand dunes. These look much like sand waves except that the wavelength is longer and they are not located within the pre-impoundment Colorado River. Instead, they are found adjacent to the river bed in low-relief overbank areas.

   mw: mass-wasting deposits. These show on the sidescan image as high-backscatter areas, which commonly have bright targets in them. Many of these deposits are found off the mouths of washes, and are thought to represent debris flow deposits associated with floods. Some, however, may just be small alluvial fan deposits. Other mass-wasting deposits are found along the base of rock cliffs. Which of these deposits post-date vs. predate the formation of Lake Mohave is unknown.

   rock: high-backscatter areas that coincide with steep cliffs on the pre-impoundment topography. Some of these areas may be semi-consolidated sediment that has not collapsed due to river undercutting prior to flooding of the area or to water saturation subsequent to the filling of Lake Mohave.

   low bs: low backscatter areas on the sidescan image that are not in the pre-impoundment Colorado River bed nor in areas identified as overbank. The origin of these areas is unknown.

   moderate: moderate backscatter areas on the sidescan image occur outside the pre-impoundment river bed and do not have the small channels that are characteristic of alluvial fan areas. In general they appear to be smooth sandy slopes to either side of the river.

   bank: steep slope that shows as either high- or low-backscatter on the sidescan image. Their location, in general, aligns with the edge of the pre-impoundment Colorado River shown on the pre-dam topographic map. In general, the banks are found off overbank and alluvial fan areas where there is unconsolidated sediment that had been eroded by the river. By contrast, where rock cliffs fringe the original river, there are no bank features.

   sw: sand waves with wavelengths of 10-30 m. They are only found on the floor of the pre-impoundment channel of the Colorado River, and are interpreted to be fluvially generated bedforms that have been inactive since filling of the reservoir. Sand waves mostly occur in areas of moderate backscatter.

   sw_large: sand waves with wavelengths of 100-450 m. They are only found on the floor of the pre-impoundment channel of the Colorado River, and are interpreted to be fluvially generated bedforms that have been inactive since filling of the reservoir. Sand waves mostly occur in areas of moderate backscatter.

   channel high: high backscatter on the channel floor includes areas with sidescan values between 180-254. In some places the backscatter is uniformly high and smooth and probably represents gravel beds. In other high backscatter places the return is more textured and may represent areas of outcrop or widely spread cobbles or boulders.

   channel moderate: moderate backscatter on the channel floor includes areas with sidescan intensity values of 51-179. The backscatter mostly is uniform, and is interpreted to represent sand.

   channel low: low backscatter on the channel floor includes areas with sidescan intensity values of 1-50. These areas usually occur along the edges of the channel often downstream of promontories. These areas are interpreted to represent mud deposits.

1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
   • David C. Twichell
   • David S. Foster

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
   

This shapefile is intended to provide an interpretation of the sidescan-sonar imagery collected in Lake Mohave.

1. From what previous works were the data drawn?

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

   (process 1 of 3)

   The completed sidescan-sonar mosaic in UTM, Zone 11, NAD83 projection was loaded into ArcView3.3.

   Person who carried out this activity:
   

   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
   

   (process 2 of 3)

   Based on variations in the backscatter of the sidescan-sonar imagery and information available from seismic-reflection profiles, polygons were drawn around interpreted features on the bottom of Lake Mohave.

   Person who carried out this activity:
   

   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
   

   (process 3 of 3)

   Using the calcapl.ave script contained in the ESRI samples directory, the AREA and PERIMETER were calculated for each polygon. These values are in units of meters, based on the UTM, Zone 11, NAD 83 projection.

   Person who carried out this activity:
   

   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
   

1. How well have the observations been checked?

2. How accurate are the geographic locations?

3. How accurate are the heights or depths?

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

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

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 S. Foster
   U.S. Geological Survey
   Geologist
   384 Woods Hole Rd.
   Woods Hole, MA 02543-1598
   

   (508) 548-8700 x2271 (voice)
   (508) 457-2310 (FAX)
   dfoster@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 in this publication 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?
   • Availability in digital form:

     Data format:	Size: 0.550

   • Cost to order the data:

Dates:

Last modified: 29-Jun-2004

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:

• <http://www.esri.com/metadata/esriprof80.html>