Mapping the Floors of Seas and Large Lakes

Pictured above: Oblique view of Crater Lake. The colored region is shaded-relief bathymetry (depth data) of the lake floor and the gray region is shaded-relief topography (height variations) of the surrounding caldera walls and Wizard Island. The distance across the bottom of the image is approximately 6 kilometers (3.7 miles). Image from J. Gardner and others.

Although many base maps of sea and lake floors have been constructed over the past century, their navigational and depth accuracies vary widely. The USGS has begun to use digital multi beam systems to map sea and lake floors with great precision. The two types of data collected include bathymetry (sea-floor depth) and backscatter (data that can provide insight into the geologic makeup of the sea floor). The new high-resolution base maps produced by the sea-floor mapping project will be used for:

• Identifying areas of erosion and deposition on the continental shelf,
• Locating pathways for movement of sediment and pollutants, and
• Locating areas of geologic hazards

Pictured above: A cartoon of multi beam receive cycle showing 60 beams spread over 150° swath.

The USGS, in cooperation with the National Park Service and the University of New Hampshire, mapped Crater Lake, Oregon in July-August 2000. The last time the lake had been surveyed was 1959, and scientific interest in various aspects of Crater Lake (aquatic biology, geochemistry, volcanic processes, etc.) has increased during the past decade. A new high-resolution base map (a digital base layer over which data may be placed) of the bathymetry was greatly needed.

Pictured above: Oblique image of floor of San Francisco Bay. Angel Island, Alcatraz Island, and the Marin Headlands are part of the Golden Gate National Recreation Area.

Pictured above: Oblique image of sea floor of Oahu, Hawaii and Pearl Harbor. The U.S. S. Arizona Memorial is located on the shoreline of Pearl Harbor.

Mapping the floor of Lake Mead

Lake Mead, in the Lake Mead National Recreation Area, was created by the building of Hoover Dam along the Colorado River. In 1999 and 2001, side-scan sonar imagery and seismic-reflection profiles were collected to map the geology of the lake floor and to map the thickness and distribution of sediment that has accumulated since completion of the Hoover Dam in 1935.

Pictured above: Houseboat rigged with SIS-1000, boomer, and fathometer was distinctive and drew curious crowds when docked.

Pictured above: Image of lake bottom.

Sediment Transport Research and Side-Scan Sonar Surveying in the Grand Canyon National Park:

In 1999, a multidisciplinary USGS team investigated sediment transport dynamics using underwater video, a current meter, and sediment sampling above Lee's Ferry, Arizona (pictured left). A successful side-scan sonar survey was then conducted along the first 240 miles of the Colorado River in Grand Canyon, from Glen Canyon Dam to Diamond Creek.