provide two new pro-slavery representatives. Alexandria's economy had stagnated as competition with the port of Georgetown, D.C., had begun to favor the north side of the Potomac, where most members of Congress and local federal officials resided. The Residence Act prohibited federal offices from locating in Virginia. The Alexandria Canal, which connected the C&O Canal to Alexandria, needed repairs, which the federal government was reluctant to fund. After a referendum, Alexandria County's citizens petitioned Congress and Virginia to return the area to Virginia. By an act of Congress on July 9, 1846, and with the approval of the Virginia General Assembly, the area south of the Potomac () was returned, or "retroceded," to Virginia effective in 1847. The retroceded land was then known as Alexandria County, Virginia, and now includes a portion of the independent city of Alexandria and all of Arlington County, the successor to Alexandria County. A large portion of the retroceded land near the river was an estate of George Washington Parke Custis, who had supported the retrocession and helped develop the charter in the Virginia General Assembly for the County of Alexandria, Virginia. The estate (Arlington Plantation) would be passed on to his daughter (the wife of Robert E. Lee), and would eventually become Arlington National Cemetery. Civil War era A portion of the Washington Aqueduct opened in 1859, providing drinking water to city residents and reducing their dependence on well water. The aqueduct built by the US Army Corps of Engineers opened for full operation in 1864, using the Potomac River as its source. Washington remained a small city of a few thousand residents, virtually deserted during the summertime, until the outbreak of the American Civil War in 1861. President Abraham Lincoln created the Army of the Potomac to defend the federal capital, and thousands of soldiers came to the area. The significant expansion of the federal government to administer the war—and its legacies, such as veterans' pensions—led to notable growth in the city's population – from 75,000 in 1860 to 132,000 in 1870. Slavery was abolished throughout the District on April 16, 1862 – eight months before Lincoln issued the Emancipation Proclamation—with the passage of the Compensated Emancipation Act. The city became a popular place for freed slaves to congregate. Throughout the war, the city was defended by a ring of Union Army forts that mostly deterred the Confederate army from attacking. One notable