The Lake Tana sub-system: Exploring the dynamics and the future challenges for sustainable development sustainable development

Lake Tana Ecosystem located in the north-west highlands of Ethiopia, has a global significance on which a huge livelihood, economic and socio-cultural groups are highly dependent. It is also one of 250 lakes identified by Lake Net¹¹ as having globally significant biodiversity (Barker, 2004). However, due to natural and human induced calamities, the Lake ecosystem is under severe threat. A recent decadal trend analysis of Lake Tana at four decade level (from 1968-2007) confirms that the water level was increasing for the first three decades and negative and declining for the last decade. The lowest depth was recorded in 2002/2003 and after that time onwards the level of Lake Tana was not able to restore to its original level. It was revealed that within the last 35 years more than 6.2% of the lake’s area was converted to other land covers (Minale and Rao, 2011).

According to Vijverberg et al. (2009) despite the limited direct human influences on Lake Tana, both wetlands and surrounding catchment area have already been seriously damaged by human activities and most of the original forest in the upper stream of the watershed has disappeared. The littoral region and wetlands of the lake are currently under severe degradation by the local inhabitants. Especially the area covered by papyrus has been decreasing recently by the ever growing human population. The local community is harvesting papyrus reed roots during low water level to use it as fuel wood. Farmers are cultivating the wetlands when the water is retreating.

In addition, Bahir Dar is a rapidly growing town, a six times population increase up to 1,800,000 inhabitants is expected in the next 50 years (Teshale, 2003). Based on the 2007 census conducted by the Central Statistical Agency of Ethiopia (CSA), the city has a total population of 221,991, an increase of 130.90% over the population recorded in the 1994 census. The current practice of Lake shore investments and discharging untreated industrial and domestic waste into the lake is causing adverse effects on the quality of the lake water and other aquatic life particularly fish species. Furthermore, pollution from agricultural sources such as fertilizers, insecticides and herbicides are recently increasing.

11 LakeNet- is U.S.-based nonprofit organization dedicated to bringing together people and solutions to protect and restore the health of the world's lakes.

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In the outflow of the Blue Nile from the lake, a large dam (Chara chara) was constructed in 1995 with additional gates added in 2001, resulting in increased silt load and turbidity of lake water and reduced water levels. The purpose of the dam is regulation of outflow from Lake Tana for downstream diversion to the Tis Abbay hydroelectric power stations located 35km downstream of the dam. Though fed by more than 40 rivers and streams, more than 93% of the water comes from Gilgel Abbay, Ribb, Gumara and Megech rivers (SMEC, 2008). The Northern and Northwest part of the Lake sub-basin is drained largely by five rivers namely, Ambagenene, Dirma, Megech (being the major tributary contributing large proportion), Gumaro and Arno-Garno which accounts for 18% of the basin. Ribb and Gumara rivers that account for 28% of the basin drain the eastern portion of the basin. Koga and Gilgel Abbay are the two major rivers draining the southern portion of the basin which accounted for 30% of the basin (MoWR, 2005). However, the future of these tributaries may not be continuous feeding of the Lake Tana. Koga Dam is already completed and functional since 2010. Construction of more major dams around the major perennial tributaries of Lake Tana particularly at Ribb, Gumara Megech, and Gilgel Abbay are underway (Figure 4.1 and Table 4.1).

Water withdrawal from the Western part of the Lake is already on track. Further feasibility studies and identification of potential lake water withdrawal at different sites are taking place. For instance, Tana-Beles power generation (estimated 460MW) and irrigation project that solely depend on diverted water from the Western shores of Lake Tana through a canal (12km) in the western part around Kunzila and connected to Beles river was inaugurated in 2010 (see Figure 4.2). The project cost of 7 billion Ethiopian Birr was fully covered by the Ethiopian government and expected to serve the country without any major interruption for the next 25 years (since March, 2010). After generating hydropower, it is also expected to irrigate 130,000 hectares of land. Generally, the project administrators and some government officials are claiming that there is no significant negative impact on the Lake’s water level and surrounding ecosystem however, the debate on the impact of those development initiatives and water withdrawal from the lake is still ongoing.

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On the other hand, a number of recent studies suggest that future development will exacerbate pressure on the lake ecosystem. There will be a significant fall of the water levels and massive wetland degradation in Lake Tana sub-basin in response to the planned water withdrawal and other development interventions (Dargahi and Setegn, 2011; McCartney et al. 2010; Setegn, 2010). If all the planned development occurs (see Table 4.1), the mean water level of Lake Tana will drop by 0.44 meters (m), and the average surface area will decrease by 30 square kilometers (km²) (i.e., 1%) and up to 81 km² (i.e., 2.6%) during some dry seasons. There will be prolonged periods of several years during which water levels will be much lower than they would be naturally. If environmental flow requirements (estimated to average 862 Mm³y-1) are maintained in the reach containing the Tis Issat waterfall, the mean water level of the lake will reduce by a further 0.37 m and the average lake area will reduce by an additional 26 km². Without careful management these changes are likely to have severe ecological and social consequences (McCartney et al. 2010).

During the course of this study an invasive weed, water hyacinth (Eichornia crassipes) which floats on water by forming a huge mass that blocks light and damages the fish breeding and feeding sites has been identified and reported around adjacent districts of Lake Tana. Researchers speculate that, if it is not controlled urgently, the weed will have a dramatic negative impact on the Lake’s ecosystem. The species reproduces very fast and enhances evapo-transpiration which eventually reduces the water volume. In due course, it might have the potential adverse effect on the Lake ecosystem and cover the whole lake surface within short period of time. Eventually it may also have negative impact on the Grand Ethiopian Renaissance Dam (also known as Grand Millennium Dam) which is currently under construction on Blue Nile River about 40 Km East of Sudan in the Benishangul-Gumuz Region of Ethiopia. With 6,000MW, the dam will be the largest hydroelectric power plant in Africa when completed as well as the seventh largest in the world.

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Figure 4.1: Lake Tana sub-basin, major rivers and planned development activities Source: Awulachew et al. (2009)

Figure 4.2: Tana-Beles hydropower and irrigation project inaugurated in 2010

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Table 4.1: Planned and implemented irrigation development in Lake Tana sub-basin

Source: BCEOM 1998; Mott MacDonald 2004; WWDSE and ICT 2008; WWDSE and TAHAL2008a, 2008b cited in McCartney et al. (2010).

Notes:* Demands estimated through crop water modeling and presented in feasibility study reports. Where a range of demands is presented this reflects alternative cropping patterns. Gross-net demand is water returned to the rivers.