Dams: Their Impacts on Coastal Environments
- Though long viewed as a benign alternative to other forms of power
generation, dams and their reservoirs have been implicated in a wide
range of environmental effects including, among many, release of toxic
mercury into food webs; emissions of greenhouse gases; severe alteration
of rivers, deltas and estuaries; increased coastal erosion; reduced
biodiversity and fisheries productivity; and changes in coastal phytoplankton
dynamics. One scientific study even estimated that the concentration
of reservoirs built at higher latitudes over the last 40 years has
caused the earth to spin faster.
- Dams and water diversions have degraded or rendered inaccessible
large areas of critical spawning habitat for salmon and ocher anadromous
fish in the U.S. The Atlantic salmon had virtually disappeared from
the Connecticut River, the largest river in New England, by 1814 due
to a dam constructed upstream in 1798. Salmon catches in Maine averaged
150,000 lbs per year during the late 1800s, but, by 1925, the species
was only found in two rivers. Hydropower development is one of the
main factors in the decimation of many Pacific salmonid populations,
though fisheries, agriculture, logging, mining, the release of hatchery
fish, and water pollution have played important roles. Snake River
adult chinook salmon, for example, declined from over 1,500,000 individuals
during the 1800s to less than 2,000 in 1994. At least 106 major populations
of west coast salmon and steelhead are now extinct, and many others
are at high risk.
- Dams and levees inhibit the transport of sediments required to prevent
coastal erosion. For example, the Mississippi River now carries only
half of its original sediment load which has contributed to the extensive
loss of Louisiana's wetlands. Prior to 1930, the Colorado River supplied
an estimated 125-150 million tons of suspended sediment to its delta
at the Gulf of California; now no sediment (or freshwater) from this
river ever reaches the sea.
- The reduction of freshwater flow into estuaries can reduce their
naturally high productivity and species diversity. This occurs via
declines in essential nutrient input, alterations to the salinity
regime, and increasing the concentrations of toxic chemicals entering
the system. Reduced flows can also result in the "drying out" of wetlands
and upstream salt water intrusion. The alteration of flood cycles
and runoff patterns also affects those biological processes attuned
to seasonal flow dynamics.
- Globally, there are approximately 39,000 large dams worldwide with
some 193,500 sq mi of land (approximately twice the area covered by
the Great Lakes) now inundated by their reservoirs. As of 1990, approximately
13% of freshwater carried by the world's rivers-and ultimately to
the ocean-was dammed: this amount is expected to rise to over 20%
by early next century.
- Virtually every river in the contiguous US. is now regulated by
dams, locks, or diversions. Currently there are more than 75,000 dams
higher than six feet in the U.S. with the reservoirs behind them covering
approximately 3% of the nation's land surface. In a given year, 60%
of the entire river flow of the US. can be stored behind dams.
- The vast majority of large dam construction occurred in the U.S.
between 1935 and 1965, a time when there was minimal concern over
the physical and biological consequences of reservoirs and alteration
of downstream flows. Impacts on numerous U.S. estuaries and coastal
environments of altered freshwater and sediment input regimes is still
largely unquantified, but in many cases is expected to be substantial.
- Dams and regulated rivers have provided various benefits for society
including generation of electricity, and the reduction of devastating
floods, as well as a controlled supply of water which can be used
for alleviating drought and a variety of municipal and industrial
demands. However, it is being increasingly recognized that dams constitute
an important component in the overall degradation occurring in estuaries,
deltas, and various nearshore environments.
- Though smaller dams continue to be constructed, few large dams will
ever again be built in the U.S. because of the few realistic sites
available, high costs of construction, and increased public awareness
of their negative effects.
- Some downstream effects of dams can be mitigated by changes in dam
operations such that normal seasonal flow rates are more accurately
mimicked. A few dams in the U.S. are now slated for decommissioning
because their environmental impacts are perceived to overwhelm any
other societal benefits. Indeed, political support for the removal
of environmentally problematic dams has increased dramatically in
the past few years.
Collier, M., R.H. Webb, and J.C. Schmidt. 1996. Dams and
Rivers - A primer on the downstream effects of dams. US. Geological
Survey Circular 1126. Tucson, Arizona.
Kowalewski, M., Avila-Serrano, G.E., Flessa, K.W., and Goodfriend,
G.A. Dead delta's former productivity: Two trillion shells at the
mouth of the Colorado River. Geology 28(12): 1059-1062.
Milliman, J.D. 1997. Blessed dams or damned dams? Nature
386: 325-327.
Rosenberg, D.M., F. Berkes, R.A. Bodaly, R.E. Hecky, C.A. Kelly
and JWM Rudd. 1997. Large-scale impacts of hydroelectric development.
Environmental Reviews 5: 27-54.
Rosenberg, D.M., McCully, P., and Pringle, C.M. 2000. Global-scale
environmental effects of hydrological alterations: Introduction.
Bioscience 50(9): 746-751.
Sahagian, D. 2000. Global physical effects of anthropogenic hydrological
alterations: sea level and water redistribution. Global and Planetary
Change 25(1-2): 39-48.
|
