Lake Cahora Basa Lake Cahora Basa is a human made reser- voir in the middle of the Zambezi River in Mozambique, which is primarily used for the production of hydroelectricity. Cre- ated in 1974, this 12 m (39 ft) deep lake has been strategically built on a narrow gorge, enhancing the rapid collection of water and creating the pressure required to drive its giant turbines. The lake’s dam has a production capacity of 3 870 megawatts, making it the largest power-producing bar- rage on the African continent. After the construction of Cahora Basa Dam, its water level held constant for 19 • Large mammals, including the once- Cahora Basa Dam is used in Mozambique, years, resulting in a near-constant release abundant buffalo, have almost disap- with some sold to South Africa and Zim- of 847 m3 (1 108 cubic yards) over the same peared from the delta babwe. Lake Cahora Basa can actually be period. Observations before and 20 years • The abundance of waterfowl has de- seen as a ‘powerhouse’ of Southern Africa, after the dam’s impoundment, however, clined signifi cantly although damage to the generators dur- ing the civil war in 1972 has kept the plant reveal the following negative impacts, some • Floodplain ‘recession agriculture’ has from functioning at its originally antici- of which may also have been caused by declined signifi cantly pated levels. The construction of the dam other dams on the river (e.g. Kariba), as • Vast vegetated islands have appeared well as the effects of the country’s 20-year has been opposed by communities down in the river channel, many of which the Zambezi River, where it reduced the civil war —but nearly all of which have been have been inhabited by people directly infl uenced by the over-regulation annual fl ooding on which they rely for cul- of a major river system: • Dense reeds have lined the sides of tivation. The Zambezi used to overfl ow its the river channels riverbanks, creating rich fl oodplains ideal • Many of the mangroves in the delta for cultivation. However, the Cahora Basa have dried out and died back • Erosion of the coastal zone has increased, probably more as a result has proved important as the major source • The community structure of the of the release of sediment-hungry of hydroelectrical power in Southern Afri- fl oodplain vegetation has changed, water from Cahora Basa Dam than of ca. In 2005, the Southern African Develop- with a substantial increase in trees changed fl ow patterns ment Community (SADC) member states • Meanders and oxbows, once a major • Water levels in the lower Shire River, agreed on a regional power generation feature of the fl oodplain, have be- the largest tributary of the Zam- initiative, which aims to boost the power come clogged with reeds and trees bezi downstream of the dam, have supply generated by Cahora Basa, as well as • Productive, fl ood-dependent grass- dropped, prohibiting navigation of building a second plant 60 km (37 miles) lands have been depleted of this river (Chenje & Jonson 2002). downstream (SADC Today 2005). grasses, the favoured food of The majority of power generated by the herbivorous mammals Reservoir behind the Cahora Basa Dam Unknown/UNEP/The Water Page 50 Challawa Gorge Reservoir Challawa Gorge Dam on the Challawa River in Nigeria is designed to release wa- ter into the Hadejia River for subsequent storage behind the barrage of the Hadejia Valley Project (Thompson 2005). The dam was built for securing supplies of potable water and irrigation for agricultural land around the state capital Kano. With 1 000 million m3 (1 308 million cubic yards) capacity, the Challawa Gorge Dam is the second-largest dam supplying water to the city of Kano. The dam is 7.8 months of construction, the dam was com- than available water. Decreasing rainfalls, km (5 miles) long and 42 m (138 ft) high, pleted (Julius Berger Nigeria Plc n.d.). possibly due to climate change have al- 3 and required 5.1 million m (6 671 mil- After completion, the Hadejia river ready reduced fl ows in the basin. lion cubic yards) of fi lling material. The system—one of the major water sources The result is growing tension between reservoir thus created has an area of 100 for the Hadejia-Nguru Wetlands located in water users and regions. This is already 2 million m (120 million square yards). The the Lake Chad basin—became more than leading to confl icts. Illustrations include project essentially consisted of a homo- 80 per cent controlled by the Tiga and the dogged opposition of the downstream geneous dam of decomposed rock. Core Challawa Gorge dams. These two dams states of Yobe and Borno to the construc- drilling performed at the dam’s center, feed the Kano River Irrigation Project, the tion of Kafi n Zaki Dam, and the incessant however, detected the existence of various Hadejia Valley Irrigation Project and the confl icts between farmers and pastoralists seams in the underlaying rock which made Kano City Water Supply (KCWS). Earlier over access to water. it necessary to construct an anti-seepage agreements to guarantee certain amounts Action is being taken to address these grout curtain. The dam also includes of fl ow from the Hadejia river system for issues. The Nigerian National Council on construction of a 30 m (98 ft) intake tower, the downstream communities living in the Water Resources established a Hadejia- intake main and a 600 m (1 967 ft) long fl oodplains, who are dependent on fl ood Jama’are-Komadugu-Yobe Coordination spillway. Since work on the dam com- recession agriculture, have so far not been Committee in 1999. Also, the Lake Chad menced as far back as 1977 and was inter- implemented (IUCN 2004). Basin Commission (LCBC) is implement- rupted in 1987, considerable erosion dam- In this arid region, the fast-growing ing a GEF-supported programme for the age occurred to the structure. This made it population and its economic activities de- integrated management of Lake Chad and necessary to scratch off the damaged layers mand a large share of the water resources. associated river systems (FAO 1997). prior to excavation and fi ll-work. After 26 Demand is approximately 2.5 times higher Hadejia-Jama’are River Basin, Northern Nigeria In Northern Nigeria, an extensive fl ood- plain exists where the Hadejia and Jama’are Rivers converge. The fl oodplain provides essential income and nutrition benefi ts in the form of agriculture, grazing resources, non-timber forest products, fuelwood and fi shing for local populations. It also helps The Kadawa River Project in the northern state of Kano includes a short-range B. Imevbore/UNEP/FAO to recharge the regional aquifer which scheme for safeguards against further drought. Here farmers are being taught serves as an essential groundwater source. new irrigation techniques However, in recent decades the fl oodplain the operational costs are included, the net and impacts on the fl oodplain downstream. has come under increasing pressure from benefi ts of the Project are reduced to US$ Full implementation of all the upstream the construction of the Tiga and Challawa 0.04 per 1000 m3. dams and large-scale irrigation schemes Gorge dams upstream. The maximum ex- A combined economic and hydrologi- would produce the greatest overall net tent of fl ooding has declined from 300 000 cal analysis was conducted to simulate the losses, around US$20.2-20.9 million. ha in the 1960s to around 70 000 to impacts of these upstream projects on the These results suggest that the expansion 100 000 ha more recently and there are fl ood extent that determines the down- of the existing irrigation schemes within plans for a new dam at Kafi n Zaki. stream fl oodplain area. The economic the river basin is effectively “uneconomic.” Economic analysis of the Kano River gains of the upstream water projects were The introduction of a regulated fl ooding Project, a major irrigation scheme benefi t- then compared to the resulting economic regime would substantially reduce the scale ing from the upstream dams and tradi- losses to downstream agricultural, fuel- of this negative balance, to US$15.4-16.5 tional use of the fl oodplain, showed that wood and fi shing benefi ts. Given the high million. The overall combined value of the net economic benefi ts of the fl oodplain productivity of the fl oodplain, the losses in production from irrigation and the fl ood- (agriculture, fi shing, fuelwood) were at economic benefi ts due to changes in fl ood plain would, however, still fall well below least US$ 32 per 1000 m3 of water (at 1989 extent for all scenarios are large, ranging the levels experienced if the additional prices). However, the returns per crops from US$2.6-4.2 million to US$23.4-24.0 upstream schemes were not constructed grown in the Kano River Project were at million. As expected, there is a direct trade- (UNEP n.d.a). most only US$ 1.73 per 1000 m3 and when off between increasing irrigation upstream 51 The Challawa Gorge Dam, completed CHALLAWA DAM in 1993, is the second largest of the 23 dams along the rivers in Nigeria’s Hadejia- NIGERIA Jama’are River Basin. Although the dam has improved the water supply for irrigation, 52 Credit: Africa Focus it has also ruptured the fragile natural balance along this stretch topsoil. These satellite images provide a comparison of the area of water. When it rains heavily, the river can break its banks and before and after the dam’s construction.