If you drive south on Interstate 81, you'll now see a sign north of Roanoke announcing you are leaving the Chesapeake Bay watershed. As you cross the divide, you'll enter the Roanoke River watershed. That river flows in a water gap it has cut through the Blue Ridge, through Smith Mountain Lake, Kerr Reservoir, and Lake Gaston where dams slow the natural current to a standstill, and across the Virginia border into North Carolina. The Roanoke River will continue downhill to the Albermarle-Pamlico Sound and Roanoke Island, the home of the "Lost Colony" planted by the English in the 1580's.
Most of Southside Virginia does not drain into the Chesapeake, and this was a factor in the slow development of the area in colonial times. Planters on the Eastern Shore, and on the west side of the bay in areas known as the Peninsula, the Middle Peninsula, and the Northern Neck, had direct access to international transportation. English ships could sail directly to wharfs in tidewater Virginia, load tobacco, and return home with minimal delay. Tobacco farmers in Lunenburg and Brunswick counties, however, had to haul their crops across several rivers, in the days when roads were poor or non-existent. The physical geography of the colony delayed settlement and reduced the value of the land sold in those watersheds, compared to the Chesapeake Bay counties.
Today, it's a good bet that more "Save the Bay" license plates are sold on the Northern Neck, or even in the Shenandoah Valley, than in the counties bordering North Carolina. None of Virginia on that border drains into the Chesapeake Bay - not even the southern part of Virginia Beach. Drive south on US17 on the eastern edge of the Dismal Swamp in the city of Chesapeake, and you'll see the same signs announcing that you are "Leaving the Chesapeake Bay Watershed" - just like the sign you see on Interstate 81 north of Roanoke...
The North River (the one west of Back Bay - there are other rivers with the same name in Virginia) runs due south to Currituck Sound. (Think the name suggests that the initial colonial-era travellers were more interested in their direction of travel to the headwaters of the river, rather than in the direction the water flowed?) Virginia's rivers further to the west (the Blackwater, the Nottaway, and the Meherrin - all of which flooded dramatically after Hurricane Floyd in September - 1999) drain into Albemarle Sound in North Carolina rather than the Chesapeake Bay.