METHODS

A sonar survey of Lake Mohave was completed on a 19-m long houseboat during April 13-29, 2002. The sonar equipment included an Edgetech DF-1000 sidescan sonar, a Knudsen 320BR dual frequency 3.5 kHz Chirp subbottom profiling system, and a 200 kHz echo sounder. Digital data were collected for 223 line-km (fig. 2). The sidescan sonar data were archived to DVD-R in Triton-Elics QMIPS format. The 3.5 kHz seismic reflection data were also archived to DVD-R in SEG-Y standard format (Barry and others, 1975). The 200 kHz bathymetric soundings were logged with navigation data using Hypack software. The survey lines are 300 m apart in the wider sections of the lake. Perpendicular seismic-reflection tie lines, spaced 1,200 m apart, were completed in half the reservoir. Time and weather constraints prevented them from being collected in the entire lake. In the narrower northern and southern areas of the lake, lines were spaced for optimal sidescan sonar coverage for mapping the reservoir floor. A more detailed site-specific survey was done in the area of Nelson's Landing in an attempt to identify deposits from a well-documented flash flood that occurred in 1974 (Glancy and Harmsen, 1975). Navigation was conducted using P-Code GPS at an accuracy of approximately ± 10 m.

The raw Knudsen 3.5 kHz Chirp seismic profile data, which were processed in the field to produce digital images of the profiles, were converted to match-filtered, amplitude-detected traces using SIOSEIS seismic processing software. The processed data were plotted to PostScript files using Seismic Unix software (Cohen and Stockwell, 1997). The PostScript output, in turn, was converted to JPEG images, which were used for quality control and initial interpretation.

At the USGS Woods Hole Science Center, the processed SEG-Y data were analyzed using SeisVision (Landmark Graphics Corp.) seismic interpretation software. The software allows the seismic profiles to be viewed and interpreted in their correct geospatial orientation.

The sidescan sonar data were processed to construct a digital mosaic of the lake floor. These data were demultiplexed, corrected for slant-range distortions and signal attenuation using techniques summarized by Danforth and others (1991). The individual lines were mapped to their proper geographic location using software described by Paskevich (1996). The georeferenced images were then combined into a composite digital mosaic using PCI remote sensing software (Anonymous, 1992). The techniques for generating the digital mosaic are summarized by Paskevich (1992). The completed mosaic has a pixel size of 2 m and is in UTM, zone 11, projection (WGS-84 ellipsoid). The sidescan sonar mosaic was imported to ESRI's ArcView GIS software where the interpretation of the sidescan sonar imagery was completed.