Bathymetry is the measurement of the depth of large bodies of water. Bathymetry data can be stored in a number of formats including point soundings, isobaths (contours), and grids. The bathymetry mosaic dataset is a grid coverage of water depth generated from point soundings. The grid structure is a regular array of cells referenced to the Universal Transverse Mercator (UTM) coordinate system. Each grid cell represents water depth for a discrete 20- x 20-meter area. The mosaic is a continuous grid covering Washington marine waters, Oregon waters as far south as Seaside, and the Columbia River upstream to Bonneville Dam. Canadian marine waters are included for southern Vancouver Island. The dataset was funded by the Puget Sound Ambient Monitoring Program (PSAMP). Primary bathymetry grid processing was performed under contract by Environmental Consultants (ECI), Portland, Oregon.