Issues accompanying dam removal

One of the issues accompanying dam removal is the potential impact of the large volume of sediment that accumulated behind some dams over time. This so-called "legacy sediment" is commonly very fine grained and contains stored nutrients and organic matter, among other possible pollutants. When dams are destroyed, efforts must be made to avoid a large flux of this sediment downstream as the newly released river cuts down to its natural base level. This requires careful engineering and significant funding. Merritts and Walter (2010) have suggested that most rivers and streams in eastern Pennsylvania and Maryland run through floodplains and levees of legacy sediment, not natural river valleys, created by the plethora of small dams built to impound water for hydropower in the past. And, even now, some large dams serve as a buffer against sediment transport that would create broad mudflats and high turbidity in coastal bays such as the Chesapeake Bay. Conowingo Dam in Maryland is one such structure that is estimated to trap as much as 50% of the sediment carried by the Susquehanna River before it empties into Chesapeake Bay. There are concerns that this dam only has a bit more than a decade of useful life before it is filled to the brim with sediment, thus imperiling Chesapeake ecosystems (Langland, USGS, 2012). For an example of the engineering and costs of removing a dam, check out the San Clemente Dam removal project on the Carmel River in central California.

Figure 13. Map of the lower 48 states showing locations of dams removed from 1936 to 2013. On the American Rivers website (www.americanrivers.org/.../dams/dam-removals-map/), the locations are clickable to show information on each dam removed.
Source: AmericanRivers.org Used with permission. All rights reserved.

Figure 14. Blow up of eastern U.S. portion of the dam removal map.
Source: AmericanRivers.org Used with permission. All rights reserved.

Many dam removal projects are proposed, but await funding from federal sources—Congress must appropriate funds. Projects on the Snake River and the Klamath River in the west remain controversial, but at least in the case of the Klamath, the Department of the Interior has recommended removal to allow salmon to reinhabit this river. Figure 9 illustrates the consequences of waiting for funding of dam removal using the Olmstead Dam example.

Figure 15. The costs of waiting to remove and replace dams once approved using the Olmstead Dam and Locks system on the Ohio River. Lambrecht, Bill (16 September 2013). "Costly Dam, Illinois dam costing billions more than expected as delays mount". St. Louis Post-Dispatch. Retrieved 9 March 2014.
Source: Army Corp of Engineers