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When rivers are dammed, physical and ecological impacts on waterways and their floodplains are unavoidable. The effect can be clearly seen in the Republic of Korea, where multipurpose dams built for flood protection, water supply and hydro-energy generation obstruct almost every major river. The damage is most evident downstream, where dams have shifted river gradients and dramatically altered flow regimes. In some cases impacts on available floodplain habitats have been profound, hitting both river biology and local communities.
Keen to mitigate such outcomes, the Korean Government has initiated numerous river restoration projects since the 1990s designed to improve physical characteristics of river channels and improve water quality.
Yet the effect of such good work has been hampered by a lack of knowledge about the ecological consequences of flow regime changes. Missing have been methodologies to quantitatively determine the effects of dam operations on downstream habitat and ecological conditions. Those methodologies must be flexible enough to accommodate the varied conditions found throughout Korea’s river basins.
To address the deficit, a team from eWater CRC has been working with the Korean Water Management Corporation (K-Water) in a partnership arrangement. K-Water is charged with developing a national integrated water management strategy, initially focussed on environmental flows in the Geum River Basin (GRB). The team comprised researchers from many disciplines, including hydrology, geomorphology, aquatic chemistry, water quality, algal ecology, fish ecology and vegetation ecology.
The aim was to achieve a much firmer understanding of the likely consequences of establishing an environmental flow regime suitable for fish.
Work was underpinned by an assessment of the hydraulic habitat requirements of the black shiner Pseudopungtungia nigra, an endangered fish species in Korea. The purpose of the work was to help highlight how changes to the Geum River’s flow regime have altered Pseudopungtungia nigra’s preferred habitats.
Poor Ecological Condition
The third largest river basin in Korea, the GRB runs to the west of the central section of the Korean peninsula and spans 9,835.3km2, almost one tenth of the country. The Geum River flows into the Yellow Sea.
The first multipurpose dam in the basin, the Daecheong Multipurpose Dam (DMD), was built in December 1980 and is located 150km upstream from the river estuary. Featuring a unique hybrid concrete gravity and rock fill construction, the dam has a total storage capacity of 1,490Mm3. The Yongdam Multipurpose Dam (YMD), constructed 21 years later, diverts the Geum River water resources into the Mangyeong River basin area to address municipal and industrial water needs. This concrete-faced rock-fill dam stores 815Mm3 and features a 21.9km long diversion tunnel.
At present the river has relatively poor water quality and its ecological systems, including riparian vegetation, are in an equally poor state. Sedimentation is a continuing problem.
To explore the dam’s effects, the team collected available data on the GRB’s ecological condition, including water quality, fish abundance, wetland water birds, and other criteria. These were used to model the river’s ecosystem responses to flow regime change.
The results are clear: the nature of the fish community in the Geum River has changed under the impact of the dams. Fish are always a useful pointer to the health of aquatic ecosystems, and the DMD has clearly altered the nature of the fish fauna to be found upstream and down.
For instance between 2002 and 2004 five introduced fish species were found downstream of the DMD. Meanwhile up to 30 native fish species may have gone missing, many of them Korean natives particularly sensitive to alteration of the flow regime and/or habitat structure. Fewer sensitive riffle benthic species can be found in the reaches downstream of DMD. The results suggest sensitive riffle-benthic species have been lost thanks to the reduction or alteration of suitable habitat conditions.
Equally, in the relatively brief period since construction of the YMD, the nature of the fish community upstream and downstream has altered, suggesting that this dam may have had a similar negative impact on species composition.
Meanwhile time series analysis shows that since flow regulation, low flows (90th percentile) have increased and high flows (10th percentile) have decreased. The effect is most noticeable in springtime (March, April and May in Korea) when flow variability at the SuTong gauging station below YMD is significantly lower than it was before the dams were constructed. The researchers believe the loss of high flows and increased low flows during normally dry months, in combination with reduced flow variability, has significantly altered habitat structure and brought about significant ecological consequences.
The research fed creation of some ‘fundamental concepts’ about the impacts of dams, which are forming the basis for a pilot assessment of the ecological outcomes of experimental environmental flows, relative to reference or ‘baseline’ conditions. To be useful and successful, the e-flows must accommodate any physical, regulatory, legislative and operational constraints in the GRB and be readily understood by dam operators and stakeholders alike.
The conceptual model and associated integrated numerical modelling for hydraulic fish habitat assessment of the Geum River basin will guide desktop and field studies, modelling and scenario evaluations.
The researchers warn it would be unwise to undertake any remedial actions to restore the ecosystem, and particularly the fish community, until all the factors are better understood. However, they suggest the conceptual and modeling approaches developed for a future whole-of-river assessment can be used for fish, vegetation, and water quality along the main channel as well as the major tributaries of the Geum River.
The research team is also confident the proposed method can be integrated efficiently with the existing tools such as SPEARS (Species at Risk Index) for the river management.
“The intent is to produce a suite of biological parameters that define ecological structure and function and then to conduct habitat condition assessments and estimate the flows required to maintain suitable habitat for each section of the river” (Park Sangyoung, Jeongkon Kim, Ick Hwan Ko, Angela Arthington, Gary Jones and Kyung Taek Yum 2010).
The research was partly supported by a grant from the Sustainable Water Resources Research Center and involved application of eWater CRC's river catchment expertise and toolkit to GRB.
The researchers are confident the resultant GRB conceptual model, used in conjunction with the eWater integrated toolkit, will allow river managers to isolate the physical and biological effects associated with dam operations. They also hope it will assist development of sustainable river management strategies. The GRB conceptual model is providing a framework for structuring, analyzing, and quantifying the impacts of altered hydrologic regimes below dams on the river’s ecosystem and on an endangered fish species. The conceptual model can also be helpful in identifying reference scenarios for comparative purposes, and in facilitating evaluation of future restoration strategies.
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