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The Akosombo Dam on the Volta River in Ghana has cut off the supply of sediment to the Volta Estuary, affecting neighbouring Togo and Benin, whose coasts are now being eaten away at a rate of 10-15 metres per year. A project to strengthen the Togo coast has cost US$3.5 million for each kilometre protected. The story is the same on coastline after coastline where dams have stopped a river’s sediments.
Hydrological Effects Dams also change the pattern of the flow of a river, both reducing its overall volume and changing its seasonal variations. The nature of the impacts depends on the design, purpose and operation of the dam, among other things. All parts of a river’s ecology can be impacted by changes to its flow.
Estuaries A river’s estuary, where fresh water meets the sea, is a particularly rich ecosystem. Some 80% of the world’s fish catch comes from these habitats, which depends on the volume and timing of nutrients and fresh water. The alteration of the flows reaching estuaries because of dams and diversions is a major cause of the precipitous decline of sea fisheries in the Gulf of Mexico, the Black and Caspian Seas, California’s San Francisco Bay, the Eastern Mediterranean and others. The regulation of the Volta River in Ghana by the Akasombo and Kpong dams has led to the disappearance of the once-thriving clam industry at the river’s estuary, as well as the serious decline of barracuda and other sport fish.
Changes to Flooding The storage of water in dams delays and reduces floods downstream. River and floodplain ecosystems are closely adapted to a river’s flooding cycle. The native plants and animals depend on its variations for reproduction, hatching, migration and other important lifecycle stages. Annual floods deposit nutrients on the land, flush out backwater channels, and replenish wetlands. It is generally recognised by biologists that dams are the most destructive of the many abuses causing the rapid disappearance of riverine species. About 20% of the world’s recognised 8,000 freshwater species are threatened with extinction.
The Mary River needs a number of small flood events each year (2-4m high) for water quality, fish migration, turtle nesting, just to name a few. None of these small flood events will get through to downstream areas, let alone all the way to Maryborough and Hervey Bay. The negative impact on water quality and ecology will affect everyone in the catchment. It will only be the one in ten year big flood event which could overtop the big dam and flood the downstream river for a short period.
Information courtesy of the IRN, Mary River Action Group, and Sunshine Coast Environment Council (SCEC).
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Effects of Dams on River Systems |
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‘Downstream impacts have been the Cinderella of debates about dams: unrecognised, misunderstood and underestimated by planners. One reason for this is that they occur in remote areas, far from the dam site, and are all too easily ignored. Even when recognised, downstream impacts are daunting in their complexity in space and time...The downstream impacts of dams are complex, and have knock-on secondary and tertiary impacts on aquatic and floodplain ecosystems.’
World Commission on Dams Thematic Review 1.1: Downstream Impacts of Large Dams January, 2000 |
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It is a fact universally acknowledged: by reducing the flow of water from a river one changes the landscape it flows through. A dam holds back sediments, especially the heavy gravel and cobbles, and the river, deprived of its sediment load, seeks to recapture it by eroding the downstream channel and banks, undermining bridges and other riverbank structures. Riverbeds are typically eroded by several metres within the first decade, and the damage can extend for tens or even hundreds of kilometres.
The International Rivers Network (IRN) gives the example of the Aswan High Dam on the Nile River. Before it was dammed this River carried about 124 million tons of sediment to the sea each year, depositing nearly 10 million tons on the floodplain and delta. Today, 98% of that sediment remains behind the dam. The result has been a drop in soil productivity and depth, among other serious changes to Egypt’s floodplain agriculture. The Aswan Dam has also led to serious coastal erosion, another problem stemming from the loss of sediments in a dammed river. |
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