sentences:  It was named for Rock Island, an island in the Mississippi River now known as Arsenal Island.The Rock River (which the Sauk and Fox Indians called Sinnissippi meaning "rocky waters") flows from Whiteside County and points further east and north and joins the Mississippi River at Rock Island.The Sinnissippi Mounds, dating from the Hopewell period and on the National Register of Historic Places are upriver at Sterling in Whiteside County.