OF 2004-1247 HOME

Disc Contents

Introduction

Data/GIS

• Getting Started
  
  
• Interpretation
  
  
• Navigation
  
  
• Seismic-Reflection
  
  
• Sidescan-Sonar
  
  

Metadata/Data Catalog

References Cited

Contacts

Interpretation:

The sidescan-sonar mosaics were used to interpret the surficial geology of the reservoir floor. This interpretation is based on the backscatter intensity as well as the patterns of the backscatter. In this project, high-backscatter values appear as light or white, while low-backscatter values are black or dark gray. A complete description of the criteria used to interpret the various features is presented in Foster and others (2004). This report presents the polygon shapefile representing that interpretation.

What follows is a series of images showing the interpretive layer of Lake Mohave, working from north to south. Clicking on any image will open a larger area of the same area. The legend for the interpretive layers is available as a separate image and at the bottom of this page with a more descriptive text.

Interpretation definitions:

The following is a more detailed description of the sidescan interpretive layer. The text in parentheses is the text contained in the INTERP attribute of 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 (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.

bank (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.

high backscatter channel (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.

low backscatter channel (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.

moderate backscatter channel (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.

dunes (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.

low backscatter post-impoundment (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 backscatter post-impoundment (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.

mass-wasting deposit (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.

overbank (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.

rock (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.

reworked alluvial fan deposit (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.

sand waves (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.

large sand waves (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.