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Dams
and Rivers: Human and Ecological Consequences
These
temple destroyers, devotees of ravaging commercialism, seem to have a perfect
contempt for Nature, and, instead of lifting their eyes to the God of the
mountains, lift them to the Almighty Dollar. Dam Hetch Hetchy! As well
dam for water-tanks the people's cathedrals and churches, for no holier
temple has ever been consecrated by the heart of man.
- John Muir,
Hetch Hetchy Valley
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In this lecture period, we wish to learn:
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How common are dams?
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What functions do dams serve?
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What are the harmful effects of dams?
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What are the alternatives?
1. How common are
dams?
How common are dams? Unfortunately
for the ecological health of rivers, dams are all too common. There are
nearly 2,000,000 small dams in the US, and 75,000 large ones (six feet
or higher). The number of large dams is well known – the Army
Corps of Engineers keeps a data base. The number of small dams is only
an estimate.
Dams affects a staggering fraction
of U.S. river miles. For comparison, there are 3.5 million miles of rivers
in all of the U.S. In the US, there are 600,000 river miles impounded behind
dams. In contrast, only 10,000 river miles (not even half of 1%) are permanently
protected under the National
Wild and Scenic Rivers System. Only one river -- The Yellowstone –
has more than 600 miles in length that is still undammed. Most major rivers
of the Northern Hemisphere are dammed, usually at multiple sites, resulting
in major fragmentation of river habitat.
The pace at which large dams are
built is staggering. Through the 1980s, about 500 large dams were built
each year. In recent years, the pace of dam-building has slowed somewhat,
to about 300/year, and dam-building has shifted to developing countries.
Why are dams built? Dams serve many
functions, which we will discuss shortly. Without water, many areas cannot
be farmed or settled. Numerous well-written books (see especially those
by Reisner and Worster) describe the damming of the Colorado, Columbia,
and other great rivers. Great dams, such as Hoover Dam that forms Lake
Mead, were enormous technological achievements, which not only allowed
deserts to be farmed and settled, but also represented an unprecedented
triumph of human endeavor over nature. In another era, the construction
of great dams was a source of national pride (and the Hoover dam today
is a popular destination for tourists). But for another view, consider
this quote from ecologist Philip Fearnside, concerned about dam development
in the Amazon: "Like the pyramids of ancient Egypt, these massive public
works demand the effort of an entire society to complete but bring virtually
no economic returns".
The economics of dams is a complicated
subject, one we will not resolve here. But we can briefly examine the functions
of dams. This will remind us of the many reasons that at least some economic
benefits flow to some individuals when dams are built. Understanding the
functions of dams also is important to anyone who wishes to argue for dam
removal or more river-friendly dam management.
2. What are
their functions?
Some dams are built for a single
purpose. Navigation dams in the Mississippi River have the sole purpose
of maintaining water levels high enough for navigation; as flow increases,
they fold down against the river bottom. Many dams have multiple uses.
Glen Canyon Dam, which forms Lake Powell on the Colorado River, generates
hydroelectric power, and stores "upper basin" water from high runoff years
to distribute to the "lower basin" during low runoff years, equalizing
inter-annual variation in water availability. It creates a reservoir popular
for recreational boating.
Let’s examine principal functions
of dams.
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Hydropower: there are more than 2,000
major hydropower dams in the US.
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Recreational purposes (creation of lakes)
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Supply of water for human consumption
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Irrigation of agricultural lands
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Flood control
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Political pork and public works projects
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Navigation
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Tourism (eg, Hoover Dam near Las Vegas)
Hydropower provides an interesting
example of choices we face between two "bads". Global climate change is
driven by the burning of fossil fuels. We can reduce the generation of
atmospheric CO2 produced by fossil-fuel energy plants in one of the following
ways: energy conservation, nuclear power, and renewable energy sources.
But energy conservation might limit economic progress, and no-one wants
a nuclear power plant in their neighborhood, so "renewables" are a popular
choice. Renewable energy includes wind, solar, geothermal, and hydro, and
increasingly is being marketed as "green energy". But almost all "green"
energy is from dams. Hydropower accounts for about 10% of generated electricity
in the US.
Energy deregulation is an increasing
national trend, allowing consumers to choose their energy provider. Energy
suppliers use a variety of marketing approaches, and one is "green energy".
As a consumer, you will have to decide what marketing claims you believe,
and you will be able to choose between polluting the air or killing salmon.
Visit the California Hydropower Reform
Coalition to see an effort to establish criteria by which hydro-power
can be considered "green".
3. What are their harmful
effects?
Dams have many harmful
effects on rivers. They change the physical environment, altering the
variation and cycles of flow that occur daily, seasonally, and inter-annually.
Rivers are powerful engines of erosion. Over time they wear away mountains,
transport sediments and chemicals to the sea, and shape the landscapes
through which they flow. The river channel itself is in a dynamic equilibrium,
shaped by "scour" during flood events, and "fill" when flow recedes. Islands
form in larger rivers at points where transported material is deposited.
The river channel itself shifts location, wandering back and forth across
the floodplain, while the entire valley floor erodes downward, leaving
terraces that mark earlier floodplains.
All of this physical dynamism has
enormous biological consequences. Habitat diversity is maintained and habitats
are rejuvenated by the episodic change in river flow. A common effect of
dams is to regulate river flow, effectively "flat-lining" the system. Flow
becomes virtually constant year-round, as dams store flood waters, and
release it later during normal periods of low flow. Temperature can become
very constant, if the water is released from near the bottom of high dams,
because water temperature is very constant and cool at the bottom of deep
lakes and reservoirs. Sediments settle out in reservoirs, and so the river
below a dam is ‘sediment-starved". It will likely be more transparent,
and so more algal growth may occur. It will be "sediment-starved", and
so erosion and down-cutting are common.
The
natural flow regime: a paradigm for river conservation and restoration
(Poff et al. 1997) describes
the many biological consequences of altered flow regimes.
| Two of general importance are changes
in riparian vegetation, and damage to fish populations. Riparian (streamside)
vegetation is harmed because many plants depend in complex ways on variation
in flow, for replenishment of soil moisture, to keep out competitors, to
disperse seeds, and to favor seedling survival. Fish are affected by disruption
of cues to the spawning cycle, by loss of habitat, by proliferation of
non-native species that benefit from regulated by flow, and by simple blockage
of passage.
The effects of dams on salmon has
been a lightening rod for dam-related issues. At present (1999-2000), the
dams and salmon of the Snake River (fig 1.) are one of the hottest environmental
issues today. Go to Snake
River Salmon page for further discussion. |
Figure 1: Lower Granite Dam on the Snake river.
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4. Summary
Many dams, both large and small,
block rivers, fragmenting and altering their habitat and impairing the
health of river ecosystems. Dams serve numerous purposes, of which irrigation,
hydropower, and navigation are particularly important. Today, concern for
global warming together with energy deregulation is leading to a marketing
of hydropower as "green" energy. Dams harm river ecology by changing their
physical and chemical dynamics, which in turn affects the biota, including
riparian vegetation and fish.
The harmful effects of dams on river
ecosystems can be addressed by removing dams, changing the ways dams operate,
or my mitigating dam effects, e.g. by fish ladders and barging young salmon
around dams. The current political controversy surrounding the Snake River
dams and salmon illustrates how heated the debate can become.
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