Main Ideas
- Today about 800,000 dams operate worldwide, 45,000 of which are large—that is, greater than 15 meters tall.
- Most were built in the past century, primarily after World War II.
- Dams control flooding, and their reservoirs provide a reliable supply of water for irrigation, drinking and recreation.
- In the U.S., where hydropower dams must be relicensed every 30 to 50 years, the rate of dam removal has exceeded the rate of construction for the past decade or so.
- Some 800,000 dams exist around the world, but small ones—and even some large ones—are increasingly being removed so rivers and streams can recover
- Ecologists are learning, however, that removing or lowering dams takes a great deal of careful planning and active intervention because sometimes the dams confer environmental benefits, such as holding back toxic sediments or blocking the progress of invasive species.
- Before decommissioning a dam on Fossil Creek in Arizona, managers poisoned exotic fish and airlifted native species to safety. Such strategies could prove key to the success of future dam removal projects.
- In some cases, dams have blocked invasive species from moving upriver and into zones above the dam.
- The release of sediments trapped behind a dam’s walls can choke waterways, muddying the environment and wiping out insects and algae, which are important food for fish.
- When engineers removed the Fort Edward Dam on the Hudson River in 1973, concentrations of PCBs rose in downstream fish and remained high for many years; even today the striped bass fishery remains closed because of high levels of PCBs.
- Ecological insights and more flexible engineering, it seems possible to think of the world’s waterways as ultimately fulfilling their promise for all parties—from plants to people.
Author's Main Point |
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My Reflection |
Dams have advantages and disadvantage. The way they work is that they are
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