Three Gorges Dam: An Ecological Perspective of Natural Flow and Dam Operations.
In a series of publications, including one in a recent issue of Frontiers, Wu et al.
(2003a,b, Frontier 2004 2:241-248) reviewed the potential impacts of the Three Gorges
Dam (TGD) in China and suggested the future direction of ecological research related to
TGD. Wu et al. clearly delineated the effects of damming on spatial patterns of
landscape via inundation and eloquently put forward research questions related to habitat
fragmentation. In addition to their delineation, I would like to point out two related
issues that deserve serious attention, the dam operation and the consequence of
alternating temporal patterns of flow, particular downstream of TGD.
The ecological impacts of a dam depend as much on dam operation as on the physical
structure of dam. Undoubtedly, the physical structure sets constraints on water flow and
many ecological processes. The dam interrupts the river continuum and the connectivity
of populations, trophic structures, and ecosystem processes. However, it is the dam
operation that controls the regime of water distribution, both spatially and temporally. It
is the operation that determines the exact quantity of flow at any point of time, and
therefore the magnitude, frequency, duration, timing and pattern of flood pulses
downstream.
Most river ecosystems have evolved out of natural flood pulses. Flood pulses directly
influence water quality, sediment deposition and transport, physical habitat conditions
and connections, and energy sources in the river. Microbes, plants and animals that live
in the river and affiliated wetlands are highly adapted to the rhythms of natural
hydrologic variability. These organisms are responsible for much of the water
purification, decomposition, and nutrient cycling that occurs in the river. Natural flood
pulses maintain the diversity of native species, the integrity and health of river and
riverine wetland ecosystems, and the ecosystem services that the river supports (Junk et
al. 1989, Poff et al. 1997).
Traditionally, dam operations seldom take into account ecological consequences and
ecosystem services, and instead focus on maximizing economic benefits of hydrological
alternation, such as power generation and flood control. Such operations create different
artificial pulses upstream and downstream, alter or reverse the flow temporal patterns,
and decouple flow pulses with climate seasonality and the phenological rhythm of biota.
As a consequence, abundance of native species declines, extinctions occur, fish
productivity and biodiversity decrease, exotic species invade and expand, food web
structures become distorted, and nutrient cycling and fluxes are changed. These impacts
severally damage the ecosystem functions and services (Poff et al. 1997). Further, once
ecosystems are degraded, it is usually prohibitively costly and often impossible to reconstruct the functions and services. Recently, the trend in operations has started to
change. An increasing number of dam authorities started to consider minimizing
ecological damages and incorporate ecological restoration into the operations.
Here, I would argue, TGD authority should recognize that the control and reduction of
ecological damages and ecological restoration are legitimate goals. The maintenance and
restoration of natural flood pulses should be included in the operation plan. TGD
managers seem yet to realize the ecological importance of the natural flow regime. TGD
is the largest dam in the world. Downstream of TGD is arguably the most important
economic region of China, populated by hundreds of millions of people. The wellbeing
of these people and the economy of region rely on the ecosystem services provided by the
river and the natural flow regime. The ecological demands must be considered among
the competing demands in dam operation. An ecologically sound operation schedule not
only can reduce and control some seemingly inevitable damages, but also can help to
partially restore the already damaged ecosystem. Land use changes, such as urban
development, agriculture, and deforestation, have already altered the flow regime of
Yangtze River and severely damage the river ecosystem in the past. In contrast, an
ecologically ignorant operation plan will cause much more and extremely severe
damages.
Making and operating an ecologically sound flow management plan will create great
challenges and opportunities for ecologists. Managers will need ecologists to provide
quantitatively accurate scientific information. Meanwhile, the operation brings about a
great grand-scale experiment downstream. The planned hydrological manipulation may
allow scientists to design controlled studies to investigate how hydrology links to ecology
at the regional and habitat scales. Currently, uncertainty is high and accuracy is low in
our quantitative understanding. The system is very large and complex. The dam
managers, ecologists, engineers, hydrologists, and stakeholders will need to work
together and develop an adaptive management strategy, of which key elements include
spatially explicit hydro-ecologic models and systematic monitoring of representative
populations, community structures, ecosystem fluxes and landscape patterns. As TGD is
already constructed, the debate “to build or not to build” is long over. Now, the question
“how to operate the dam with the least ecological damage” becomes critical and needs to
be addressed as early as possible.
Reference
Dong
Junk, W.J., P.B. Bayley, and R.E. Sparks. 1989. The flood pulse concept in riverfloodplain systems, pp. 110-127. In: D.P. Dodge (ed.) Proceedings of the Interna- tional
Large River Symposium. Can. Spec. Publ. Fish. Aquat. Sci. 106.
Poff, N.L., Allan, J.D., Bain, M.B., Karr. J.R., Prestegaard, K.L., Richter, B.D., Sparks,
R.E. and Stromberg, J.C. 1997. The natural flow regime: A paradigm for river
conservation and restoration. BioScience. 47: 769-784.
Wu, J., J. Huang, X. Han, Z. Xie, and X. Gao. 2003. Three-Gorges Dam: The Largest
“Natural” Experiment of Habitat Fragmentation? Science 300:1239-1240.
Wu, J., J. Huang, and X. Han. 2003. Three-Gorges Dam: Risk to ancient fish. Science
302:1149-1150.
Wu, J., J. Huang, X. Han, X. Gao, F. He, M. Jiang, Z. Jiang, R. B. Primack, and Z. Shen.
2004. Three Gorges Dam: An Ecological Perspective. Frontiers in Ecology and the
Environment 2(5): 241-248.