Ghana is well endowed with water resources, but the amount of water available changes markedly from season to season as well as from year to year. Also the distribution within the country is far from uniform with the south-western part better watered than the coastal and northern regions. However, availability of water is decreasing owing to rainfall variability (climate change), rapid population growth, increased environmental degradation, pollution of rivers and draining of wetlands. All of Ghana's rivers drain southwards to the Gulf of Guinea. The Volta River, with a catchment area within Ghana of nearly 70% of the country, is by far the largest river draining the entire north, centre and east of the country. The remaining rivers, all in the south and southwest, drain about 30% of the country.
The major sub-basins of the Volta include the Black and White Volta Rivers, the Oti River and the Lower Volta, including Lake Volta. The South-Western Rivers System comprises the Bia, Tano, Ankobra and Pra Rivers, while the Coastal Rivers System is made up of Ochi-Amissah, Ochi-Nakwa, Ayensu, Densu and Tordzie/Aka Rivers.
The Volta River basin is shared with Cote d'Ivoire, Burkina Faso, Togo, Benin and Mali. The Bia is shared with Cote d'Ivoire, while the lower reaches of the Tano River also form part of the boundary with Cote d'Ivoire.
Impoundments and reservoirs have been constructed for hydropower generation, water supply and irrigation. At Akosombo, 100 km from the mouth of the Volta, the first Volta hydroelectric dam was constructed in 1964, which has created one of the largest man-made lakes in the world, covering an area of about 8,300 km2. A smaller impoundment, the Kpong Headpond, covering an area of about 40 km2, was completed in 1981, when another hydroelectric scheme was commissioned at Kpong, 20 km downstream of Akosombo. Other important impoundments are the Weija and Owabi Reservoirs on the Rivers Densu and Offin, respectively. In addition to these, the only significant natural freshwater lake in Ghana is the meteoritic crater lake, Lake Bosumtwi.
Surface water quality considerations are becoming increasingly important due to mining activities, urban and industrial pollution problems and agricultural development. Reliable data on water quality is of importance for proper management and thereby the protection and development of surface water resources for the future. A further important ongoing concern requiring appropriate hydrological data is the current and future development of urban drainage in a number of Ghana's major cities, for which flood and storm runoff data is needed for proper planning and design.