Introduction to Stream Restoration - Hydro-Environmental Analysis: Freshwater Environments - page 232

The American Fisheries Society (AFS) recognizes that dams and associated aquatic communities
provide many important societal beneits but that river blockages may cause adverse environmental
impacts and societal costs. The net costs and beneits of dams should be compared to traditional val-
ues that were affected by altered habitat and ecology. American Fisheries Society believes that dam
removal can be a legitimate alternative to mitigate the adverse environmental effects of dams and their
operation. Decisions about dam removal should rely on the best available scientiic information give
full, objective consideration to local costs and beneits and broader, regional considerations. The AFS
supports dam removal when it is determined that both: (1) the beneits of dam removal outweigh the
costs associated with societal, cultural, environmental, economic, engineering, and technical issues;
and (2) dam removal is the best approach to restore ish habitat and the ish populations and isheries
they supported. Removal decisions should be selected with full stakeholder involvement.
When deemed to be the preferred alternative, dam removal should minimize impacts to aquatic and
riparian resources. The AFS recognizes that adverse impacts to isheries and impounded ecosystems
are an unavoidable consequence of dam removal, but a well designed removal can minimize short-term
impacts. Over the longer term, removal is often warranted where temporary impacts are outweighed
by the long-term beneits of dam removal. When the decision to preserve or rebuild a dam is made,
effective and eficient ish passage facilities should be included at the structure to mitigate dam induced
fragmentation of the river ecosystem and resulting impacts to aquatic communities.
Since 1999, more than 185 dams have been removed across the country, while in 2005 alone,
56 dams in 11 states were removed or slated for removal in order to restore the natural river condi-
tions (Clark 2007). The dams that are removed are often obsolete or past their design life, often
requiring repair or rehabilitation, are of only marginal value (such as for hydropower and other
uses), or ecologically destructive where restoration of the ecological function is of greater beneit.
The removal of dams may result in considerable temporary and long-term changes in terms
of lows, sediment transport, water quality, and stream habitat and usually requires a substantial
environmental and public safety review before implementation. For example, in addition to safety
concerns with the dam breaching, a consideration for dam removal is the quantity and quality of
the sediments behind the dam. Dams act as sediment traps so there may be substantial accumula-
tions of sediments that must be removed to preclude them from being washed downstream once
the dam is removed. In addition, the sediments may potentially be contaminated, impacting their
removal and disposal. An evaluation is required to determine the most appropriate methods for the
removal and/or transport of the sediments. There are often considerable structural materials that
must be removed and disposed of, such as dam materials, gates, and other hardware such as utility
connections. The construction of temporary access roads may be required. Following the removal
of the dam, restoration of the former reservoir basin as well as tailwater areas is usually required.
While dam removal is complex, and often controversial, it is an important alternative for river
restoration. Restoration of the unregulated low and sediment transport regimes following dam
removal have resulted in increased biotic diversity through the enhancement of preferred spawning
grounds or other habitats and the reappearance of rifle-pool sequences, gravel, and cobble areas.
Fish passage has been another beneit of dam removal (Bednarek 2001). Many of the impacts of
dams and the factors associated with their removal are summarized in a special issue of BioScience
on “Dam removal and river restoration” (Hart and Poff 2002).
8.4.7 d efLectorS
Delectors are one of several instream structures that have been used extensively to create instream
habitats for ish as part of restoration projects (Shields 1983; Biron et al. 2004, 2009 and others).
Delectors concentrate the low either vertically or horizontally. They may be used to increase
the speed of low locally to create areas of differential scour and deposition, such as to create or
enhance pool-rifle areas, or to delect the low away from erodible banks. Delector types include
(Figure 8.19; Heaton et al. 2002; Stormwater Manager's Resource Center http://www.stormwater-
center.net/):