GOLDEN AGE OF DAMS
The building of dams in the U.S. began as early as the 1600s and became more prolific during the industrial rev- olution. But the “golden age” occurred between 1950 and 1980. Dams were built from the headwaters to the head- of-tide, thus prohibiting the passage of
Without the free-flow- ing nature of a river, the impoundment water column stratifies like a pond or lake, water temperatures
increase, algae blooms occur, and dissolved oxygen lowers.
diadromous species such as rainbow smelt, river herring, shad, and American eels. Only small residual diadromous
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populations that could tolerate a saline spawning environment or had small freshwater stretches below the dams were able to survive.
What happens over time after a dam
is built? Almost immediately the im- poundment aging process starts. Without the free-flowing nature of a river, the impoundment water column stratifies like a pond or lake, water temperatures increase, algae blooms occur, and dissolved oxygen lowers. These conditions lead to poor water quality and resulting fish-kills to native species accustomed to riverine condi- tions. Within the impoundment, sedi- ment and debris become trapped, destroying the river bed habitat and preventing both sediment and nutrient transport further downstream. Aquatic vegetation grows throughout the im- poundment as it becomes shallower, further exacerbating poor water quality conditions. Pollutants can become trapped within the sediments, which can affect fish, wildlife, and public health throughout the food chain. Changed conditions downstream of
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a dam include riverbed scour, altered temperature regimes, diminished water quality, altered flows, and, of course, blocked fish passage. Both migratory and resident fish species are restricted or prevented from accessing necessary life cycle habitat. Upstream, freshwater resident fish may also be affected over time by inbreeding, potentially eliminating native popula- tions. Fish passage systems can help, but often do not allow all species to pass. Seasonal systems will not serve all the species, and the varying swim- ming capabilities between species require different passage systems. There are more than 75,000 dams recorded with the National Inventory of Dams, and only a minority have effective fish passage systems. Across the country, attitudes are changing as dams age, repairs are needed, and owners no longer want the responsibility or liability. River restoration has gained momentum, with more than 700 dam removals nationwide. In New Hampshire, 12 dams have been removed, two removals
TIDE
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