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How can we use flowing freshwater to generate electricity?
dams!
grand coulee dam
-LARGEST dam in the U.S.
-8 yrs. to complete (1934-42)
-built on the Columbia River, in Washington
hoover dam
-2ND LARGEST dam in the U.S.
-5 yrs. to complete (1931-36)
-built on the Colorado River, in Nevada
why do we use dams?
-dams produce hydroelectric power.
-reservoirs behind dams can be used for recreation.
-dams are used to control water for use in irrigation and in cities.
-dams help to control downstream flooding.
pros of damns
-provides water for year-round irrigation of cropland
-provides water for drinking
-reservoir is useful for recreation and fishing
-produces cheap electricity
-downstream flooding is reduced
cons of dams
-flooded land destroys forests or cropland and displaces people
-large losses of water through evaporation
-downstream cropland and estuaries are deprived of nutrient-rich silt
-risk of failure and devastating downstream flooding
-migration and spawning of some fish are disrupted
how does a dam work?
flowing water spins a turbine, triggering a generator which makes electricity.
one major con of dams
dams can reduce downstream flow so much that water barely reaches the ocean. some major rivers now run dry during dry seasons, which has implications for wildlife living in, or near, these bodies of water.
fish ladders
designed to help fish get back to spawning waters.
anadromous mating habit
fish swim from the ocean back to freshwater to spawn. (salmon)
catadromous mating habit
fish swim from fresh water back to the ocean to spawn. (many eels)
chinas three gorges dam
-1.4 miles long.
-electric output is 8X the electrical output of the Hoover Dam.
-displaced 1.2 million people, and who knows how many species met their death.
-built over a seismic fault and already has small cracks.
The Wild & Scenic Rivers Act of 1968.
-signed by President Lyndon B. Johnson
-ended dam building era of the 20th century
-Dams are being removed to reestablish ecosystems, and because they have outlived their usefulness, but their removal could cause toxic sediments to be released
factors to consider before dam construction
-water availability at different times of the year
-effects and occurrence of drought or excessive precipitation
-pollution loads from surrounding land use (fertilizer run-off?)
-habitat needs of aquatic life (& spawning practices of some fish!)
-values people of that region place on wildlife, fisheries, recreational use of river basins
