The current population pressure, inappropriate cultivation practices, forest removal and high grazing intensities on forests, wetlands, rangelands and marginal agricultural lands leads to unwanted sediment and stream flow changes that mainly impacts the downstream human and natural communities. Forests and bush are cleared, and wetlands are encroached to create space for human settlement, roads construction and to satisfy wood fuel energy demands. Similarly, pastoral areas are subjected to growing human and livestock populations, leading to land degradation, soil erosion and to an increase in the load of non-point pollutants. Landscape disturbance over many decades, and the resulting increase in soil erosion and sedimentation is the dominant cause of the ongoing eutrophication in many of the lakes in eastern Africa. Increased sedimentation in the rivers and lakes has many impacts. For example, it has altered some aquatic habitats and communities, contributed to increasing eutrophication, abetted the proliferation of algal blooms and water hyacinth reduced the amount of dissolved oxygen, etc. This paper outlines some of the problems created by increased sedimentation within the East African Great Lakes basin, and provides some possible solutions to the mitigation of sediment flux through integrated sediment management approaches.

Introduction

Over the past three decades or so, the lakes and rivers of eastern Africa (Figure 1) have come under increasing and considerable pressure from a variety of interlinked human activities such as over fishing, species introductions, industrial pollution, eutrophication, and sedimentation. The current population pressures, inappropriate cultivation practices, forest removal and high grazing intensities on forests, wetlands, rangelands and marginal agricultural lands has led to unwanted sediment and stream flow changes in the rivers and lakes of the eastern Africa region (Botero, 1986; Magunda and Majaliwa, 2000). In most of the lakes, it appears that much of the sediment deposition occurs in the littoral zone, precisely where most of the lake!s biodiversity is concentrated. Increased sedimentation will alter aquatic habitats, while increased water turbidity as a function of sediment load and sediment deposition thwart algal growth, which may have profound effects upon other components of the foodweb (Odada et al., 2003). While the impact of industrial, municipal and domestic discharges are poorly understood, some studies suggest that pollution has altered, in some areas, the composition of phytoplankton communities (Cocquyt et al., 1991). Of many serious environmental threats that the east African lakes face, the excessive loads of sediment and nutrients caused by deforestation and erosion in the watershed, industrial and urban pollution are amongst those that need to be urgently addressed. The environmental degradation is driven by the need to sustain the growing population (United Nations Environment Programme-UNEP, 2004).

Catchment disturbance and sedimentation

Deforestation and agriculture

The overall effect of deforestation/change of plant species because of population pressures is increased sediment loading to rivers and lakes in the basin (Swallow et al., 2002). The sediment loads from such areas are normally high in nutrients and organic matter (Ffolliot and Brooks, 1986). Forests and bush are cleared, and wetlands are encroached to create space for human settlement, roads construction and to satisfy wood fuel energy demands (Odada et al., 2004). Similarly, pastoral areas are subjected to growing human and livestock populations, leading to land degradation, soil erosion and to an increase in the load of non-point pollutants.

The deforestation process is essentially complete within the Burundi and northern D.R. Congo portions of the Lake Tanganyika watershed (Cohen et al., 1993). Bizimana and Duchafour (1991) have estimated soil erosion rates in the deforested and steep sloping Ntahangwa River catchment in northern Burundi to be between 20 and 100 t ha−1 yr−1. Increased deforestation and consequently erosion in the Lake Tanganyika catchment has caused an increase in suspended sediment entering the rivers and the lake (Bizimana and Duchafour, 1991; Cohen, 1991; Tiercelin and Mondeguer, 1991). In Lake Malawi basin, catchment disturbance through land clearance has resulted in greatly increased sediment loads and runoff (Table 2; Bootsma and Hecky, 1999; Calder et al., 1995; Eccles, 1984; Tweddle, 1992). The rapid increase in population and in agriculture has led to the large-scale disappearance of the original woodland so that many parts of Malawi now experience a shortage of firewood (Eccles, 1984). The forest cover is estimated to have declined by 13% between 1967 and 1990 (Calder et al., 1995).

Most of the farming systems in the lake basins are associated with slash and burn land management practices (Odada et al., 2003). However, there are also large-scale farms of coffee, tea, cotton, rice, maize, sugar and tobacco (Ntiba et al., 2001) where the use of agrochemicals is increasing in the Lake Victoria basin. Due to poor farming practices, there is associated soil loss from the productive lands that are transported to the rivers and consequently to the lakes. The soil loss poses the greatest threat to sustainable agricultural production and negatively impacts on water quality (Kasweswe-Mafongo, 2003). For example, approximately 46% of the 3,516 km2 Nyando River basin (or 1,624 km2) of Lake Victoria has experienced severe soil physical erosion, resulting in soil physicochemical degradation or soil nutrient deficiencies of one form or another (Swallow et al., 2002). In the Mara, Mwanza and Kagera regions, clearing of forests has resulted into deforestation, a dominant feature in most parts of the area where land is left bare following the expansion of settlements, livestock keeping, and agriculture (Hongo, 2000). Poor agricultural practices, deforestation and soil erosion, amongst others, have undermined the region!s bioproductive systems and its economic base while contributing to the greenhouse effects (cf. Ottichilo et al., 1991).

Mining and industry

Several chemical pollution studies have detected low levels of trace metals and pesticides in the water, sediments, plants, and fish species of the lake (Wandiga and Onyari, 1987; Wasswa, 1997; Ejobi et al., 1994; Kasozi, 2001; Wandiga, 1981; Kituyi et al., 2001; Ruud, 1995). However, though the concentration levels are below the acute toxicity level they may be of concern to the food chain (Wandiga et al., 2002). The increase of small scale gold mining in Tanzania in particular (gold mining also takes place in Kenya) is leading to some contamination of the local waterways by mercury which is used to amalgamate and recover the gold; some traces of heavy metals such as chromium and lead are also found in the lake, although the problem has not yet reached major proportions. There have been serious accidental spills, e.g. of DDT in Kigoma harbour (Alabaster, 1981) and fuel oil leakages in Mpulungu harbour, although DDT is no longer used except near the shoreline in the Zambian side of the lake (Cohen et al., 1996). Suspended sediments can be a major conveyance for the heavy metals, hydrocarbons and pesticides, but their role needs to be quantitatively assessed.

Urban and rural settlements

Poor planning, maintenance and inadequate investment in municipality waste-water treatment systems have contributed to the increased untreated effluent discharge. For example, if the present treatment plants in Kisumu would perform optimally, the BOD could be brought down by 50% (Scheren et al., 2000). High inputs of organic sediments to rivers and lakes are also due to urban and rural settlements lacking sewers (Table 3), and poor animal husbandry in the context of large livestock populations. The situation is similar in the other lake basins, both large and small.

Biomass burning and air particulate transport

Most of the farming systems are associated with slash and burn land management practices, resulting in the release of greenhouse gases and nutrients to the atmosphere. It is thought that the nutrients released to the atmosphere may be an important source of nutrients to the large lakes of eastern Africa, and that it is enhancing the rate of eutrophication in the lakes (Bootsma and Hecky, 1999; UNEP, 2004). In the southern parts of the East African Great Lakes region, land clearing using uncontrolled large fires is proceeding at an alarming rate (Cohen et al., 1996). However, more studies are needed to quantify this effect.

Wetland disturbance

Many human exploitation activities in the East African wetlands are potentially sustainable; however, an expanding and accelerating trend is towards large-scale drainage and conversion of large tracts of land for agriculture. For example, the Yala valley swamp in Kenya (along Yala river) is host to many fish species, plants, invertebrates and birds as well as mammals and reptiles. Currently, part of the swamp has been reclaimed for agricultural use, and more recently, the government of Kenya plans to drain the swamp for agricultural purposes (Aloo, 2003). In Lake Tanganyika basin, intense cultivation of floodplains, grazing by cattle and burning during the dry season (Hughes and Hughes, 1992) has led to significant loss of wetland e.g. in Burundi. Cattle grazing and agricultural activities (sugar and cotton growing under irrigation) are common in the marginal areas of the Shire Swamps in the lower part of the course of the Shire River that supports one of Malawi!s most important fisheries (Hughes and Hughes, 1992).

Sedimentation impacts on the aquatic environment

Evidence for enhanced sediment loading of the lakes

Landscape disturbance has been documented from sediment cores in Lake Victoria and appears to have begun in the 1930s (Figure 2; Verschuren et al., 2002). From satellite imagery, it has been observed that nutrient-rich sediment plumes originating from agricultural runoff and the low-lying, deforested riparian zones and other areas surrounding the lake are feeding the water hyacinth (Wilson et al., 1999). Similar impacts have been observed in other lake basins, For example, Yuretich (1979) observed in the Lake Turkana that sediment plumes up to 100km long extend southward from the Omo River delta during flood seasons, partly due to increased soil erosion on the banks of the affluent rivers due to human activities (Haack and Messina, 1997; Waktola, 1999). As a result, the sediment load is high, in common with other arid environments (up to 1600 t km−2 a−1), and delta construction is rapid (Frostick and Reid, 1986). Consequently, at the Omo River entrance to Lake Turkana, there has developed a highly complex and spatially fluctuating floodplain and delta (Haack and Messina, 1997). The rapid and extensive growth of the Omo River delta over the past few decades reflects significant changes in sedimentation. Today the lake color, especially in the north, is brown because of sedimentation (Haack and Messina, 1997); the euphotic zone is about 6m, and the lake is always turbid (Kallqvist et al., 1988).

Significant stretches of Lake Tanganyika!s coastline have been transformed from rocky substrates to mixed rocky/sandy substrate or entirely sandy substrates (West, 2001), and river deltas, such as the Rusizi River delta, have been prograding. It appears that much sediment deposition occurs in the littoral zone, precisely where most of the lake!s biodiversity is concentrated. In some cases more than one meter of sediment has accumulated at these sites (West, 2001). Analyses of sedimentation rates from 14C dated cores (Tiercelin and Mondeguer, 1991) confirmed the high sediment impact in the northern basin with the southern and central basins receiving < 1,500 mm/1,000 years and < 500 mm/1,000 years respectively, compared to the northern basin which received about 4,700 mm/1,000 years. More recent studies by Sichingabula (1999) and Kakogozo et al. (2000) show that annual lakewide sediment input into Lake Tanganyika is enormous (Table 1). In addition, three significant landslides that occurred near Gatororongo (estimated at more than 11,280 tons at this site alone) show that, especially in the rainy season, significant amounts of sediment can be introduced into the lake without transiting through rivers (West, 2001). The nutrient and sediment loading to Lake Malawi from its influent rivers has likely increased by 50% within the past few decades with a few rivers such as the Linthipe, Songwe and Dwanga accounting for much of that increase (Hecky et al., 2003; World Bank, 2003).

Impacts on water quantity and water quality

Increased sedimentation and reduced flow have hampered navigation within the rivers (UNEP, 2004). Most of the rivers are now polluted and are unsafe for use as potable water—this is prevalent mostly in rivers that flow through urban settlements and along the largely unplanned coastal settlements that lack proper sanitation infrastructure (UNEP-IETC, 2003; UNEP, 2004). Due to the lake!s long residence time, pollution (including sediments) resulting from the effects of human activities and development in its catchment is potentially catastrophic to the lake!s water quality, economic fish stocks and overall biodiversity (Duda, 2002; West, 2001). In addition, it is less likely that damage can be reversed once it occurs (Spigel and Coulter, 1996).

Eutrophication

In Lake Victoria, for example, domestic biological oxygen demand (BOD) exceeds industrial loads in all regions (Scheren et al., 2000). Nearly half of the lake floor (Lake Victoria) currently experiences prolonged anoxia for several months of the year, compared to the 1960!s when anoxia was localized and sporadic (Talling, 1965, 1966; Hecky, 1993) and this is largely attributed to increased nutrient inputs. Blue green algae have been observed floating in the middle of the lake, indicative of lake-wide eutrophication. High nutrient inputs have abetted the proliferation of water hyacinth. For instance, in September 1998 the water hyacinth mat covered 400,000 hectares of the Kavirondo Gulf in Kenya. In the same year, four fifth!s of Uganda!s shoreline was covered by the hyacinth mat. Its spread disrupted fishing activities, transportation, and threatened the functioning of various lakeshore-based installations such as water purification and hydroelectric power plants (Twongo, 1996).

Aquatic ecosystem change

Changes in the aquatic ecosystem are evident in all of the lakes, but are particularly prominent in Lake Victoria, and these, to varying degrees, may be partly attributed to increased rates of sediment and associated nutrient influx. In Lake Victoria, algal blooms have increased since the 1960!s (Mugidde, 1993). The filamentous and colonial blue green algae, known for causing hypoxia conditions that occasionally lead to fish kills is now very dominant in the lake (Kling et al., 2001). Algal concentrations are three to fivefold greater on the surface waters today than in the 1960!s, reflecting higher rates of photosynthesis (Mugidde, 1993). In consequence, dissolved silica concentrations in the water column have plummeted to 10 per cent of their 1960s values (Hecky, 1993; Verschuren et al., 2002). Enhanced denitrification has lowered the N:P ratio and blue green cyanobacteria have replaced diatoms as the dominant phytoplankton in the lake (Hecky, 1993; Verschuren et al., 2002). A study in Lake Victoria (Uganda) has shown that, in the vicinity of the water hyacinth, fish species number, biomass and diversity are reduced, the former two very significantly (Willoughby et al., 1996). This is because proliferation of the water hyacinth leads to reduced oxygen levels, and hence reduced floral and faunal diversity (Kudhongania et al., 1996). It, however, provides a protective habitat for some of the endangered haplochromine species, hippopotamus, crocodiles, snakes, and snails and mosquitoes which carry bilharziasis.

Although studies are needed to ascertain the type and extent of change in the different lake basins, studies in Lake Tanganyika have revealed some of the impacts of increased sedimentation. There is evidence that excessive sedimentation has reduced the diversity of nearshore fishes (Cohen et al., 1996). For example, in studying ostracods across a variety of habitats that were lightly, moderately or highly disturbed by sediment in Lake Tanganyika, Cohen et al. (1993) found that ostracods from highly disturbed environments (both hard and soft substrate) were significantly less diverse than those from the less disturbed environments with differences in species richness that ranged from 40–62 percent. Species richness for deepwater ostracods followed the same general pattern, though the differences were not as great. Some populations of cichlids and molluscs have gone locally extinct during the past 30 years (West, 2001). For example, Alin et al. (1999) have noted that sediment inundation of lacustrine habitats has reduced species richness and density of molluscs, and the species richness of ostracods. The changes in ostracod (Cohen et al., 1993) and benthic algal (O!Reilly, 1998) communities impact on ecosystem structure and function, affect all levels of the aquatic food chain. These data suggest that sediment input may have already had an important role in altering ostracod community structure. Benthic algae productivity studies show that sediment inputs from deforestation probably reduce the amount of available habitat for colonization, decrease the nutrient value of the food source, and reduce the feeding efficiency of the primary consumers (O!Reilly, 1998).

The cyprinid fish Opsaridium microlepsis, which is endemic to Lake Malawi and is one of the major commercial species in its northern and central regions (Hughes and Hughes, 1992) has now been largely eliminated from Malawian waters through a combination of siltation and fishing pressure (Cohen et al., 1996). In addition, water level changes in the floodplains can have marked impacts on fish catches because the floodplains act as very productive nursery areas (Ribbink, 2001).

Riparian wetlands

The full extent of wetland use and impacts is not well known. For example, satellite imagery suggests that impacts on Lake Victoria!s wetlands are substantial, based on observed erosion from shoreline zones (Wilson et al., 1999). Sediment influx to the lakes is increased when wetland areas are reduced or destroyed. The Omo River delta, in an arid area that is sparsely populated compared to the other lake basins with more mesic climates, offers two interesting perspectives. The delta has grown by about 380 km2 between 1973 and 1989 as seen from landsat images, largely as a result of increased sediment flux from upstream areas due to human activities in the Omo River catchment, and decreased water flow (both a function of anthropogenic effects and changes in the hydrological cycle). The delta!s wetland expansion is potentially maintaining or increasing the biodiversity of fauna and flora, both locally and regionally, but is also attracting permanent human populations, most likely in conflict with flora and fauna (Haack and Messina, 1997). Human activities are, therefore, directly threatening the ecology, biodiversity and system function of wetlands through either increased erosion of the wetlands themselves, or increased sediment flux to the wetlands, which results in their further expansion, and this, in turn, attracts human exploitation.

Sediment management

Consequences of non-intervention

The various forms of sediment pollution have had diverse impacts: siltation threatens the production capacity of the hydroelectric power plant and operations of irrigation systems; reduced water quality and quantity increases has increased the costs of water treatment and water supply, respectively; agricultural productivity is facing a decline due to soil loss and inadequate water supply for irrigation. If there no intervention measures are taken soon, increased agricultural activities and land degradation along the rivers will result in increased suspended solids being transported to the lakes. There would be ramifications for economy: sediment blanketing and increased turbidity influences changes in benthic and pelagic biodiversity, affecting the fisheries resources particularly in the riverine, shallow water and deltaic areas where subsistence fishermen fish. Health would be affected as a result of an anticipated increase in microbiological and chemical contamination from organic wastes (settlements, agro-chemicals).

Conservation strategies

Conservation efforts should be directed at minimising soil erosion. Settlements within the area need to be planned and have proper sanitation facilities, better animal husbandry needs to be incorporated, and clean water sources of potable water, such as groundwater should be explored and harnessed to serve the communities.

Data requirements

In order to effect sound sediment management strategies, a proper scientific understanding of the environmental issues is required, based on targeted analysis of environmental data. Such data required include: climate and hydrological data, land use and cover changes and associated sediment sources, transport pathways and flux measurements, and historical (natural) perspectives (e.g. lake and river sediment core analyses) to establish baselines and trends in the absence of long term instrumental data records.

Management requirements

The management of environmental problems in the lake basins have been inadequate, in part due to addressing the problems from either sectoral or mono-disciplinary perspectives that fail to recognise the complex interplay of environmental factors, and hence these problems are not addressed in a holistic context. Management requirements therefore include: integrated river basin management with legal provisions for environment, water and sediment management policies and strategies; education and participation of stakeholders in the management programmes; and increased use of knowledge-based strategies for sediment management.

Specific needs

The specific issues that need to be addressed are many and include the following: conservation of forests, trees and wetlands; protection and stabilisation of steep slopes; implementation of riparian buffer zone programs; treatment and reduction of municipal and industrial effluents; reduction in flood potential through, for example, protecting against developments in areas prone to flood hazards; and, other measures for reduction of sediment and nutrient transport, especially of phosphorus, into the rivers and lakes.

Conclusions

The sediment impacts in Africa!s transboundary river and lake basins have wide and far-reaching consequences that often negatively impact the environment and livelihoods of the basin inhabitants. Some of these impacts are habitat modification and biodiversity changes, water pollution, loss of soil physical properties and texture with consequent reduction in agricultural productivity, increased atmospheric aerosol loading, amongst others. The control of sediment flows is thus a critical issue that needs to be addressed in the context of integrated water resources management and by adoption of participatory approaches that involve all stakeholders from the grassroots to the policy level. These approaches should be firmly anchored on sound scientific investigations and best-practices principles to ensure that the instituted controls, mitigation and adaptation measures are effective and sustainable in the long-term for the benefit of current and future basin inhabitants.

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