by Chevron, which as part of its oil exploration and development program dredged the White Nile shoals and established navigational beacons from Kosti to Bentiu. Manmade features such as the growing number of dams also restricts use of the river. As of 2011 (before the secession of South Sudan), Sudan had 4,068 kilometers of navigable rivers overall, but only 1,723 kilometers were open throughout the year, making river transport minimal. The most important route used to be the 1,436-kilometer stretch of the White Nile from Kosti to Juba (known as the Southern Reach), which provided the only generally usable transport connection between the central and southern parts of the country. Such river traffic ended in 1984 when the SPLA regularly sank the scheduled steamers, but it began to recover following the signing of the CPA in 2005. Transport services also ran at one time on tributaries of the White Nile (the Bahr al-Ghazal and the Jur River) to the west of Malakal. These services went as far as Wau but were seasonal, dependent on water levels. They were discontinued during the 1970s because vegetation blocked the waterways, particularly the fast-growing water hyacinth. In early 2003, a tributary of the White Nile east of Malakal, known as the Sobat River Corridor, reopened, improving the distribution of food aid in the region. On the main Nile, a 287-kilometer stretch from Kuraymah to Dongola, situated between the fourth and third cataracts and known as the Dongola Reach, also has regular service, except during the low-water period in February and March. Since 1981 the government has tried to remedy past neglect and requested foreign assistance to dredge the rivers, improve the quays, and provide navigation aids. The River Transport Corporation (RTC) operated as a parastatal from 1973 until 2007 when two private companies, the Nile River Transport Corporation and the Sudan River Transport Corporation, took it over. Before that, the latter companies were run by the Sudan Railways Corporation essentially as feeders to the rail line. Another parastatal, the joint Sudanese-Egyptian River Navigation Corporation, operated services between Wadi Halfa and Aswan, but service often was disrupted by political tension between Egypt and Sudan. Since the privatization of the RTC, other private operators started providing services. There were six private companies operating river vessels in 2009. The government began in 2003 to expand the Sea Ports Corporation in order for it to manage river services