California's Channel Islands have a lengthy archaeological record, spanning roughly 13,000 calendar years. However, relatively little is known about cultural developments during the Middle Holocene, resulting in a substantial gap in our understanding of the prehistory of California. Our research at CA-SRI-667, a large dune site with three components dated between about 6,200 and 4,300 cal B. P., demonstrates significant environmental changes occurred through time. Faunal remains and artifacts from the site document the decline of a local estuary, intensive dune building episodes, and the presence of relatively mobile human settlement systems. These data illustrate the dynamic nature of Middle Holocene human settlement and subsistence strategies, and associated environmental changes on California's Channel Islands.