and other small boats plied the Blackwater upstream of Franklin in colonial times (and still do today), in the steamboat era, navigation was restricted to the river downstream of Franklin. South of Franklin, the Blackwater River forms the border between Southampton County and Suffolk. About nine miles (14 km) south of Franklin, on the border between Virginia and North Carolina, the Blackwater River is joined by the Nottoway River, forming the Chowan River, which continues south to Albemarle Sound. The boundary between Virginia and North Carolina runs west from the Atlantic Ocean, striking the Blackwater River about a half mile (1 km) north of the confluence with the Nottoway River. From there the state line follows the Blackwater River south to the confluence, then continues west. The watershed of the Blackwater River contains a portion of three cities and five counties of Virginia: the cities of Franklin, Petersburg, and Suffolk, and the counties of Isle of Wight, Prince George, Southampton, Surry, and Sussex. Natural history The Blackwater River is, as its name implies, a blackwater river — its waters are clear, dark, acidic, low in nutrients, and tannin stained. Several environmental factors cause the blackwater condition. The many forested wetlands along the river include Streamhead Pocosins and Bald Cypress and Tupelo Swamps. The Blackwater is known for its pocosins, including the rare type called streamhead pocosin. The only two streamhead pocosins known in Virginia are on the Blackwater River. Bald Cypress and Tupelo Swamps occur along many streams of the Atlantic Coastal Plain. Most were logged, thus old-growth examples are rare. The Blackwater River Preserve protects one of the oldest bald cypress and tupelo swamp forests in the South, with trees up to 800 years old. The largest known water tupelo in the United States, according to the National Register of Big Trees, is in Isle of Wight County. It stands tall with a circumference of . Another stand of old-growth bald cypress and tupelo forest, known as Dendron Swamp, was recently designated a Natural Area Preserve by the Virginia Department of Conservation of Recreation. Another rare type of forest found along the Blackwater River is a type called Longleaf Pine and Mixed Pine Flatwoods. The forests were once common in southeastern Virginia, but today are restricted, in Virginia, to a few small sites on the east side of the Blackwater River. Near the town of Zuni, Virginia, one stand