Use of decision support systems to improve dam planning and dam ...

and nutrient-depleted. In contrast, water released from near the bottom is usually cold, oxygen-depleted,
and nutrient-rich that is often high in hydrogen sulfide, iron and/or manganese. These differences in water
quality can have significant impacts on downstream flora and fauna.
Eutrophication of reservoirs may occur as a consequence of large influxes of organic material and/or
nutrients. In many cases these are the consequence of human activities in the catchment. Eutrophication can
result in blooms of blue-green algae (Figure 2.3) which as well as being aesthetically unpleasant are, in some
instances, toxic and can cause oxygen depletion at the bottom of the reservoir and increased pH and oxygen
in the surface waters.
2.5 Impacts on sediment
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Figure 2.3. Algal bloom in the Hartebespoort Reservoir in South Africa.
Reservoirs reduce flow velocity and so enhance sedimentation. Consequently, over time they fill with sediment.
The rate at which a reservoir fills depends on characteristics of the catchment in which the dam is located,
human land-use practices and the operation of the dam. The reduction in sediment in rivers downstream of
dams can result in increased erosion of river banks and beds, loss of floodplains and degradation of coastal
deltas. In some circumstances increased sedimentation (aggradation) occurs because material entering from
tributaries cannot be moved by the regulated flows.
The Lower Nile River has been affected by the construction of two dams. The Aswan (1902) and the
Aswan High (1970) dams were built to control the floodwaters of the Nile, to generate electricity and to
provide year-round irrigation. These dams have significantly increased the amount of land available for
agriculture and lengthened the agricultural year. Other changes, however, have been less favorable. The
Aswan High Dam traps more than 98% of the sediment transported by the Nile, reducing the deposition of
nutrient-rich silts onto downstream floodplains and the delta (Stanley and Warne 1998). Offshore fisheries
that depended on nutrients from the Nile have drastically declined (Nixon 2004). The delta constitutes
two-thirds of Egypt’s arable land, but because it is being starved of fluvial sediments, it is steadily yielding to
coastal erosion at a rate of around 5-8 m per year (Stanley and Warne 1998). In addition, reduced soil fertility
means that farmers have to apply increasing amounts of fertilizer to replace the nutrient-rich sediments
trapped in the reservoir.
Source: Matthew McCartney