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Rivers and streams are the circulatory systems of our watersheds. They transport freshwater, nutrients and sediment from land to sea; and link and sustain virtually all types of ecosystems. In Maine, rivers support a tremendous diversity of life, and form migration corridors for all sorts of aquatic species and terrestrial wildlife. Plant, insect, invertebrate, and fish species abound in healthy river systems in well-protected watersheds. Approximately one half of our nation's drinking water supply comes from rivers.
There are thousands of dams and impoundments located throughout the watersheds of Maine, as well as flow diversions and canals which remove water from natural riverbeds for hydropower generation, irrigation, and other purposes. Many of these dams were built before the 1900's during our nation's industrial revolution and have long since been abandoned by their owners as no longer useful. Paper companies, hydropower generators and sometimes federal, state, or municipal agencies own these dams, but often many private owners have died, transferred ownership, or have moved away, leaving old structures to deteriorate and sometimes collapse.
Among all the development threats to rivers in our watersheds, dams rank as the first and most persistent alteration to riverine ecosystems. Summarized below are some of the most easily recognized detrimental effects caused by dams.
Dams reduce river levels. By diverting water for power, dams remove water needed for healthy in-stream ecosystems. Stretches below dams can be completely devoid of water with the exception of minor leakage, totally destroying aquatic and riparian habitat.
Dams slow the flow of rivers. By decreasing the rate of water flow many fish species such as salmon depend on seasonal high flows to flush them downriver early in their lifestages, and guide them upstream years later to spawn. Fish passage structures can enable some fish to pass around a dam, but multiple dams along a river jeopardize even those species able to use such structures. In addition, stagnant reservoir pools disorient migrating fish and significantly increase the duration of the migratory pattern.
Dams alter water temperatures. By slowing water flow, most dams increase water temperatures. Other dams decrease temperatures by releasing cooled water from reservoir bottoms. Fish and other species are sensitive to these temperature irregularities, which can damage native populations.
Dams alter timing of flows. By withholding and then releasing water to generate power for peak demand periods, dams cause downstream stretches to alternate between no water and powerful surges that erode soil, vegetation, and flood or strand both aquatic and mammalian species. These irregular releases also destroy natural seasonal flow variations that trigger growth and reproductive cycles in many species.
Dams fluctuate reservoir levels. Peaking power operations can cause dramatic changes in reservoir water levels - often up to 40 feet - which degrade shorelines and disturb fisheries, waterfowl, and bottom-dwelling organisms. Fluctuating levels cause methylization of organic mercury which accumulates in the food chain and can affect fish, wildlife, and humans that utilize different species as a food source.
Dams hold back silt, debris, and nutrients. By slowing flows, dams allow silt to collect on river bottoms and bury fish spawning habitat. Silt trapped above dams accumulates heavy metals and other pollutants. Gravel, logs, and other debris are also trapped by dams eliminating their use downstream as food and habitat.
Dam turbines cut up fish. Following currents downstream, fish are drawn into and cut up by power turbines. In some cases when fish are trucked or barged around dams to avoid turbines, they experience increased stress and disease and decreased homing instincts when returning to the rivers to spawn.
Dams increase predator risk. Warm, murky reservoirs often favor predators of naturally occurring species. In addition, passage through fish ladders or turbines can injure or stun fish, making them easy prey for flying predators like gulls and herons.
Further examination of the damages caused by the existence of dams in Maine reveals an endless litany of very specific impacts to hundreds of plant, animal and aquatic species. Combining the cumulative effects of these damages within ecosystems throughout Maine's watersheds, there exists a scenario that provides myriad resource impacts that need to be addressed. Workable long term watershed protection strategies need to include consideration of dam removal.
| Want to know more? Check out these clean river resources... | |||
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 | American Whitewater is a leading boaters-rights advocacy organization. Visit them online at: www.americanwhitewater.org | Maine Rivers’ mission is to protect, restore and enhance the health and vitality of Maine's Rivers. www.mainerivers.org | River Network is a national leader in supporting grassroots river and watershed conservation groups. www.rivernetwork.org | RiverSmart is a national public education campaign helping Americans understand how daily water usage affects rivers. www.riversmart.org www.riversmart.org |
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