Ecological Effects of Small Dams
There are over 3000 dams in Pennsylvania, and over 100,000 throughout the United States. The great majority of these dams are small (<15 ft. height) and many of these small dams are obsolete and in poor repair. A growing number of small dams have been removed in Pennsylvania, Wisconsin, and many other states during the last two decades to reduce safety and liability risks (Bednarek 2002). Although safety and liability concerns are usually the primary factor influencing decisions to remove dams, some government agencies and environmental organizations have also proposed dam removal as a method of restoring fish passage and improving the health of stream and river ecosystems. This idea is derived in part from the extensive scientific literature documenting various effects of large (>30 ft. height) dams, along with the notion that dam removal might reverse some of these effects.
Large dams are known to impact river systems by altering several key parameters including: flow regimes and physical habitats, channel shape, sediment transport, water temperature and chemistry, and populations of algae, benthic macroinvertebrates, riparian vegetation, and resident and migratory fish (Poff and Hart 2002). The nature and magnitude of these effects are likely to depend, however, on dam size and other stream and watershed characteristics, so it is unclear whether the existing information on large dam effects is applicable to smaller dams. A better understanding of the effects of dams —particularly across a range of dam sizes— is needed to guide management decisions and maximize the effectiveness of river restoration projects.
With funding from the Pennsylvania Growing Greener Program and the Patrick Center’s Endowment for Innovative Research, scientists from the Patrick Center and the University of Delaware are studying the potential ecological effects of dam removal across a range of dam and stream/watershed characteristics using an approach based on ecological risk assessment. The study involves a regional assessment of the effects of different-sized dams on important components of the ecosystem including river channel structure, water quality, algae, benthic macroinvertebrates, fish, and streamside vegetation.
Pages featuring a gallery of small dams, links and references are also provided.