1. Need:
a. Water need is increasing with population (projected 7.3 to 10.7 billion people) and urbanization
b. Management of water resources (more efficient technology, recycling, enforcing environmental standards, higher water prices, and industries moving away from water intensive activities) uses less water
c. 50 L /person/day water is necessary (with error for geography, etc.)
d. 67% agriculture, 19% industry, 9% municipal/domestic, 5% (dry climates) for evaporation
e. 1/3 of countries in water stress are expected to face severe shortages within the century à water in unevenly distributed
f.
g. 1970s saw a boom in dam building à esp. China, US, Japan, Spain, and India
h. 2/3 large dams are in developing countries (large dam = 15m or more from foundation according to International Commission on Large Dams)
Water rich countries tend to build dams for hydropower; water poor countries tend to build them for water storage
Irrigation is largest use of water
1/3 countries rely on hydropower for over ½ of energy needs.
Dams are also built (13%) for flood protection, although they can end up worsening effects, especially when dam breaks occur.
i. Removal of dams presents problems:
i. Downstream is flushed with sediments
ii. Toxins can build up behind dams
2. Problems:
a. Transformation of river (46% of primary 106 watersheds are affected by dams)
i. Physics, chemical, and geological changes
1. Temperature, oxygen, nutrients
a. Water held at top warms; water colder after it falls (change in potential energy)
b. Supersaturation of gas (DO) after dam head (condition similar to the bends)
c. Changes often favor invasives
2. Changed flow regimes à important to many life cycles; large fluctuations in water level
a. Can set environmental flow releases to minimize effects of a changed flow regime
b. Often is 10% of annual flow (arbitrary)
c. Wetlands may dry out
d. Recharge of groundwater is diminished
e. In Africa, managed floods are used to mitigate effects on floodplains (Niger and Senegal Rivers; effective economically in Kenya) à benefits of mini-floods often are economically favorable
3. Reduced sediment and nutrient transport
a. Changes channel, floodplain, and coastal delta morphology
i. Erosive effect of waves isn't counteracted by deposition of sediment
ii. Coastline of Togo and Benin eroded 10-15 m/year because of Akosombo Dam on Volta River.
iii. Managed flood releases can help
iv. Reduces floodplain fertility?
b. Also changes in turbidity
4. Blocked migration
a. Anadromous fish
i. Sturgeon in Caspian sea must now be stocked
ii. Salmon and shad populations have died out in areas
b. Catadromous fish
c. Glochidia (on host fish) and aquatic insects
d. Often most severe ecosystem impact
e. Major cause of freshwater species extinction
f. Fish passes have low efficiency
i. Norway: 26% work with "good efficiency", 32% don't work at all
ii. Many dams have no fish passes
iii. Fish lack navigational cues like current when dams are in place
iv. Fish passes must be species AND dam specific
ii. Changes in plant life and wetlands
iii. Alterations to fauna, caused by the above two
1. Mekong River: upstream fish catches were drastically lower after dam construction; also Porto Primavera dam , Senegal and Niger rivers, Nile Delta, and Zambezi river
2. Many marine fish spawn in estuaries and deltas; freshwater flows and nutrients are important
a. Results in less food
b. Allows marine fish to invade (increased salinity)
c. Aswan High Dam reduced production on all trophic levels by reducing nutrients
d. Zambezi delta: -$10 million/year from
e. shrimp loss because of flow changes
f. compensation measures:
i. fish hatcheries and stocking programs
ii. fish passages
iii. create artificial wetlands à for shallow dams
3. Can enhance tailwater fisheries of coldwater fish, although this usually requires stocking and addition of food sources (ex. Salmanoids)
b. 261 watershed cross political boundaries
c. .5-1% storage capacity is lost each year to sedimentation
d. 20% of freshwater fish have become extinct, threatened, or endangered in recent years
i. 1 billion people rely on fish as a protein source
ii. Supply 6% of fish consumed
e. Estuarine impacts:
i. Close mouths of major rivers
ii. Salt intrustion
iii. Destruction of mangroves
iv. Loss of wetlands
f. Dams displace 40-80 million people worldwide and affect downstream communities' ability to collect riverine resources
g. Opposition to dams is common among affected communities à economic and cultural conflict
h. Reservoirs emit greenhouse gases (rotting vegetation) à must compare with pre-dam emissions to determine net effect however
i. Multiple dams can have cumulative effects à dams to the Aral sea increased salinity and pollution to the extent that fisheries were wiped out (1.25 -2.5 billion /year loss)
3. Use environmental impact assessments: many of the effects of the dams were unanticipated
a. Lack of accurate predictions, and improperly/not fully implemented plans caused environmental harm
b. Reservoir fishing can make up for economic losses downstream in some cases
c. Can change dam design, add to dam, enhance other aspects of riverine environment, or choose an alternative to a dam
i. Build dams not on main stem of a river
ii. Mitigation is largely ineffective (20% work effectively)
d. In kind vs out of kind; in basin vs out of basin compensation
e. Find out what flows are needed BEFORE dam is designed so it is flexible enough to accommodate
f. Fish have been shown to reclaim areas once dams are removed as impediments (not always, but often)
World Commission On Dams.Dams and Development: a New Framework for Decision Making. World Commission on Dams. London: Earthscan Publications Ltd., 2000. 28 Oct. 2007 <http://www.dams.org
>.