GEOMORPHIC MAP OF KITSAP COUNTY, WASHINGTON

Kitsap County, immediately west of Seattle, lies in the center of the Puget Lowland, a forearc trough mantled with late Pleistocene glacial deposits. Geologic maps of this area are needed to evaluate water resources, understand earthquake hazards, define critical areas (wetland, landslide, steep slope) for land-use planning, and better understand the habitat of endangered and threatened populations of Pacific salmon. This geomorphic map, while not a geologic map, meets many of these needs, in part because of strong correlation between surficial form and substrate in much of this young glacial terrane. The map was prepared by visual interpretation of images derived from digital high-resolution lidar topography. Though presented at 1:24,000 because of space constraints, most of the map is adequate at 1:16,000 scale or larger.

Benefits of this map include: (1) Contacts with an estimated positional accuracy of circa 5 m, mapped from a 6-ft DEM derived from 1 pulse/m² lidar surveys. (2) A uniform and relatively complete inventory of large landslides. (3) Uniform mapping of likely modern wetlands based on their near-zero slopes and characteristic surface texture that probably reflects incomplete filtering of wetland vegetation from the lidar data. (4) Delineation of beach types (accreted beach against Pleistocene glaciated surface, beach without extensive post-Pleistocene accretion or erosion; accreted beach adjacent to Holocene bluff; Holocene bluff at edge of modern tidal platform) for habitat classification. (5) Identification of alluvial flats and thus the sandy, tillable soils derived from them. (6) Recognition of upland zones where the glaciated surface (commonly underlain by silty till) was reworked by waves and currents during latest Pleistocene and/or early Holocene submergence; much of this zone is underlain by 1-2 meters of clay-free sand and gravel and thus permits relatively low-cost development of drain fields.

Mapping from the lidar DEM is faster (25+ km²/day) than field methods of comparable resolution, similar in character to mapping from air photos in unvegetated terrain but with advantages that stem from working from intrinsically-rectified digital data, and greatly superior to photo-based mapping for forested terrain such as Kitsap County.