Joseph B. Largen Roy B. Stone 1983 The drainage basins and subbasins in Duval County, Florida, are delineated on this atlas map. The county 's 840 square-mile area is drained by three major river systems; the St. Johns, 668 square miles; Nassau, 113 square miles; and St. Marys, 59 square miles. The remainder of the county is drained by a network of small streams that flow into either the Intracoastal Waterway or directly into the Atlantic Ocean. The sub-basins range in size from less than one square mile to more than 50 square miles. (USGS) application/pdf 10.3133/wri824069 en Drainage basins in Duval County, Florida reports