Dam It! No, Wait!

As world population continued to increase at a phenomenal rate, the issue of water supply will continue to be an important one. One option for providing a reliable water supply is to build dams across rivers, creating reservoirs that allow us to store excess water during high-flow seasons and use it during low-flow seasons. There are about 36,000 large dams worldwide, about half of which are in China. About 5500 are in the US – in fact, if you look carefully at a map of the US you’ll find that there are whole regions (like the southern US) with no natural lakes, but plenty of reservoirs created by dammed rivers.

It’s well known that dams have a significant impact on the environment. Reservoirs are designed for multi-use; so designers try to balance several different (and sometimes conflicting) uses such as agriculture, flood control, and recreation. When water is needed downstream its released from the reservoir, lowering water levels in the reservoirs dramatically. This sometimes results in extensive mud flats, which are aesthetically displeasing and decrease available lake fish habitat. Building the reservoirs also destroys stream fish habitat. The loss of beaches and backwater marshes as reservoirs fill is yet a third way that habitat is lost to wildlife.

A major benefit of dams is flood control. Flood-control reservoirs store part of storm water produced during heavy rains. This reduces stream flow below the reservoirs and can minimize floods in areas below the dams.

Dams are also important for producing power. In the US, for example, hydroelectric power plants provide about 15% of the power used nationally. Micro-hydropower systems, one of the world’s oldest forms of power, are designed for individual or small industrial use may become increasingly important sources of power production in the future. However, water falling over dams may pick up nitrogen gas, resulting in fish kills. Sediment which would normally move through the surface water system into a natural water body is trapped by dams, which can cause increased erosion below the dam as sediment-poor water moves downstream.

Small dams have a useful life of only about 50 years and require frequent maintenance - for example, up to 75% of dams in the US are in need of repair or replacement. Some communities, after weighing the pros and cons of reservoirs (steady water supply vs. habitat loss, for example) are opting to dismantle the dams entirely rather than spend what can be a considerable amount of money repairing the dams. Some of the benefits of dam removal, cited by the US Fish and Wildlife Service, includes restoring fisheries, cleaner river water, greater oxygenation of river water, larger buffer zone for flood control, increased recreational opportunities, and greater boat mobility downstream. The need for flood control would be mitigated by not building in floodplains. Other communities, on the other hand, feel strongly that their dams are an important and valuable resource, and that the positive effects outweigh the negative ones. It's not an easy choice, but in terms of habitat, energy, and flood control, it can be an important one.

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