The major objective of cruise IS-98 was to map the bathymetry of Lake Tahoe, California-Nevada (Fig. 1) to fulfill a commitment made during the Lake Tahoe Presidential Forum in 1997. The only existing bathymetry of Lake Tahoe, collected in 1923, was recently compiled by Rowe and Stone (1997), but the data density is inadequate for the level of scientific studies ongoing and anticipated in the near future for Lake Tahoe. Recent advances in marine multibeam-sonar capabilities now permit a cost-effective way, to precisely map the bathymetry of large areas of the ocean floor with 100% coverage. Cruise IS-98 applied this state-of-the-art ocean technology to Lake Tahoe. The newest of these high-resolution multibeam mapping systems also simultaneously collects backscatter (similar to sidescan sonar) imagery that results in a complimentary and co-registered data set that is related to the distribution of lake-floor materials and textures. The two types of maps that resulted from this cruise provide the multidisiplinary Lake Tahoe research community an unprecedented set of base maps upon which to build their studies. This report describes the high-resolution multibeam mapping system used at Lake Tahoe, outlines the data-processing steps used to produce the maps, and includes the daily log of the cruise.