• Introduction
  
• Methods
  
• Data
  
• Acknowledgements
  
• Figure Captions

Introduction

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Bear Lake (Figure 1) is a tectonic lake that has existed for at least several hundred thousand years. The lake basin

Figure 1. Location map of Bear Lake, Utah-Idaho.

is a relatively simple half graben, a spoon-shaped depression tilted toward the main fault on the east side of the lake. The U.S. Geological Survey, in cooperation with researchers from several universities, has been studying the sediments of Bear Lake since 1996 (http://climchange.cr.usgs.gov/info/lacs/index.html). The general purpose of this effort is to reconstruct past limnological conditions and regional climate on a range of timescales, from hundreds of years to hundreds of thousands of years. This research relates to a variety of human concerns, including water usage in the Bear River basin. Past work has included several coring operations, a seismic-reflection survey, sediment-trap deployments, a barge-mounted drilling operation with the GLAD800 drill rig, and a variety of other studies.

The objectives of the September, 2002 operations, preliminarily reported here, were (1) to compile a detailed bathymetric map of the lake using swath-mapping techniques, in order to provide baseline data for a variety of applications and studies, and (2) to complete a sidescan-sonar survey of the lake, providing a nearly complete acoustic image of the lake floor. Limited amounts of subbottom acoustic-reflection data (chirp) were also collected, along with samples of lake-floor sediments representative of different kinds of backscatter patterns. These surveys followed an earlier subbottom acoustic-reflection survey (1997), using boomer and 3.5 kHz systems (S. M. Colman, unpublished data).

Past seismic-reflection work has indicated that faults secondary to the east-side master fault cut the lake floor. These faults were among the primary targets of the sidescan-sonar survey. Preliminary interpretation of the data suggests that the morphology of the fault scarps on the lake floor are too subtle to be imaged by the sidescan-sonar system. However, some segments of the East Bear Lake fault at the foot of the steep eastern margin of the lake, are visible in the sidescan-sonar images. The other main targets of the sidescan-sonar survey were possible springs discharging at the lake floor. Discharge from such springs may be necessary to explain the chemistry and mineralogy of the lake sediments. A number of structures that appear to be related to spring discharge were observed in the sidescan-sonar images, and sediments at some of these features were sampled.

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