Editor’s Comments: A Downstream Opportunity

In the 1960s, a visitor to the United States stopped on a New York street to watch for a few moments the construction of a skyscraper. As he stood looking at the site, he noticed a man sketching the building and asked what he was doing. “I work for a demolition company,” the man replied, “and we make drawings of all the major buildings that go up in the city.” His company had done so for years, he said, and filed the drawings away, because having a record of the construction made it easier to decide whether to bid on a building’s eventual demolition; he figured the current building would last only about 40 years.

Whether the event happened exactly as the tourist reported it, it’s an apt commentary on the transience of structures in the US: even as we’re raising one, we’re planning on making way for the next.

One type of structure, though, often lingers longer than perhaps it should. The average dam is designed to last only about 50 years; according to the National Inventory of Dams, by the year 2020, more than 70% of the 79,000 dams in its database will be older than that. Some haven’t been adequately maintained, putting them at risk of leaking or failing. (The American Society of Civil Engineers [ASCE], in its 2005 Report Card for America’s Infrastructure, gave dams an overall grade of D.) And although most don’t pose a safety hazard, they may be obsolete for other reasons. They may also affect wildlife habitat-many block or impede salmon from reaching their spawning areas, for example-and they disrupt the transport of sediment.

It’s estimated that the number of dams being pulled down now in the US exceeds the number going up. American Rivers, an organization that promotes and in some cases funds dam removal, recently released a list of 64 dams in 14 states scheduled for destruction, and more than 700 have been removed nationwide, nearly half of them in the last decade.

Still, removing a dam is tricky. Tons of sediment is likely built up behind the dam, and releasing it has consequences. Stepped or staged removal of the dam can mitigate the effects, but more work needs to be done to study just what the short- and long-term effects are, not only on habitat-when turbidity increases and when fine sediments clog gravel beds that serve as protection for fish eggs-but also on the morphology of the streambed itself. Simply removing the dam isn’t necessarily the last step in the process.

Just as the ASCE’s periodic release of its Report Card presents some opportunities for the ESC industry, so does the trend toward dam removal. It’s unlikely that any of us was standing by, sketch pad in hand, when they were constructed, but we can still help find the best ways to manage their removal.

You can find American Rivers’ list of dam removal projects at www.americanrivers.org/2008DamRemovals.