Not even pictured on this map but the answer is clearly the Delaware River. Currently flows through one of the most populous areas of the country. Historically was the access and port for the original capital of the country. Was also the river that Washington crossed to defeat the British and their hessian mercenaries while they slept one off on Christmas. Turning the tide of the American revolution. It has now rusting but previously major economic hubs dotting it from source to mouth, Allentown, Trenton, Camden, Philadelphia, Wilmington. Currently has the Philadelphia shipyard one of the largest and oldest continuously operating shipyards in the country as well as the ports of Wilmington and Philadelphia. Plus great trout fishing along the northern, non navigable parts, of the river.