Lewis County lies within the Puget Trough section of the Pacific Border physiographic province. The west-central part of the county, an irregularly shaped area of about 830 square miles, contains the bulk of the county's population and virtually all of its ground-water development. It occupies the central part of a structural and topographic basin surrounded by well-dissected hills, which rise as much as 2,000 feet above the flood plains of the major streams. The area is drained by two major river systems, the Chehalis-Newaukum and the Cowlitz.
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Geology and ground-water resources of west-central Lewis County, Washington