What do dams mean for rivers?

In recent weeks there have been a number of items in the Australian media related to building new dams and increasing irrigation. This seems to be a good time to re-iterate the costs associated with removing water from rivers.

River scientists have devoted considerable effort to understanding what happens when we change the way rivers flow. River managers rely on this knowledge to find a balance between the different and sometimes conflicting needs for the water and services rivers provide.

Dams are built to capture and store water for cities, supply water for agriculture, generate power, and control floods. Just under 50% of the 50,000 large dams constructed globally since the 1950s support irrigation*.

Then what happens? Answering that question requires a book* but I’ll pick a few examples.

At the broadest level dams change rivers in three ways: 1. A lake or impoundment is created upstream of the wall; 2. The wall creates a barrier in the river system; and 3. The dam changes the pattern of water flow downstream.

At first glance a new lake might seem attractive. It is if you’re into water skiing, not if you are an aquatic creature adapted to living in flowing waters. In the lower Murray River for example replacing flowing river habitat with pool habitats has seriously impacted aquatic species such as the Murray crayfish (Euastacus armatus) which is now listed as endangered.

Murray crayfish (Euastacus armatus)

Murray crayfish (Euastacus armatus)

Dams are a bit like a wall across a freeway – they stop things moving down and up rivers. Fish, for example, often need to move up and down rivers to feed and reproduce. Worldwide, barriers caused by dams have lead to a decline in some important commercial and recreational fish species (such as sturgeon). Others like eels have completely disappeared from some river systems such as the Rhone in France.

Dams can also act as giant sediment traps. This is not only a problem for dam operators but for farmers on floodplains downstream of dams. The Aswan Dam on the Nile for example traps so much sediment, silt deposition on the floodplain is greatly reduced. Farmers are now applying some 13,000 tonnes of nitrate fertiliser per year to compensate.

The plants and animals that live in rivers, on their banks and on their floodplains evolved to a particular pattern of river flow (the natural flow regime). Reduced floods mean fewer opportunities for aquatic species to feed and breed on floodplains. Changes in timing of flow peaks can also trigger important life-cycle events at the wrong time of year e.g. laying eggs when its too cold.

Yarrawonga Weir is the largest on the Murray river. Picture by: Michael Bell

Yarrawonga Weir is the largest on the Murray river. Picture by: Michael Bell

The consequences are complex and far-reaching. In the southwestern catchments of the United States, for example virtually all native river fish are listed as threatened. This is largely a consequence of water withdrawal, flow stabilization and proliferation of exotic species (another issue exacerbated by dams).

For me the important thing is that all us consumers of water and hydro-power need to remember this is not a free lunch. Reduction in our demand for dams by careful water (and power) use, combined with well-informed water management can result in more needs being met by river systems.

* This post is based on information from the book: A.H. Arthington (2012). Environmental flows: Saving rivers in the third millennium. University of California Press.

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