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Restoration of riverine ecosystems is often stated as a management objective for regulated rivers, and floods are one of the most effective tools for accomplishing restoration. The National Re- search Council (NRC 1992) argued that ecological restoration means re- turning "an ecosystem to a close approximation of its condition prior to disturbance" and that "restoring altered, damaged, O f destroyed lakes, rivers, and wetlands is a high-priority task." Effective restoration must be based on a clear definition of the value of riverine resources to society; on scientific studies that document ecosystem status and provide an understanding of ecosystem processes and resource interactions; on scientific studies that predict, mea- sure, and monitor the effectiveness of restoration techniques; and on engineering and economic studies that evaluate societal costs and benefits of restoration.
In the case of some large rivers, restoration is not a self-evident goal. Indeed, restoration may be impossible; a more feasible goal may be rehabilitation of some ecosystem components and processes in parts of the river (Gore and Shields 1995, Kondolfand Wilcock 1996, Stanford et al. 1996). In other cases, the appropriate decision may be to do nothing. The decision to manipulate ecosystem processes and components involves not only a scientific judgment that a restored or rehabilitated condition is achievable, but also a value judgment that this condition is more desirable than the status quo. These judgments involve prioritizing different river resources, and they should be based on extensive and continuing public debate.
In this article, we examine the appropriate role of science in determining whether or not to restore or rehabilitate the Colorado River in the Grand Canyon by summarizing studies carried out by numerous agencies, universities, and consulting firms since 1983. This reach of the Colorado extends 425 km between Glen Canyon Dam and Lake Mead reservoir (Figure 1). Efforts to manipulate ecosystem processes and components in the Grand Canyon have received widespread public attention, such as the 1996 controlled flood released from Glen Canyon Dam and the proposal to drain Lake Powell reservoir.
Environmental change and natural hazards represent a challenge for sustainable development. By disrupting livelihoods and causing billions of dollars in damages, disasters can undo many decades of development. Development, on the other hand, can actually increase vulnerability to disasters by depleting environmental resources and marginalizing the poorest. Big disasters and big cities get the most attention from the media and academia. The vulnerabilities and capabilities of small cities have not been explored adequately in academic research, and while some cities in developed countries have begun to initiate mitigation and adaptation responses to environmental change, most cities in developing countries have not. In this thesis I explore the vulnerability to flooding of the US-Mexico border by using the cities of Nogales, Arizona, USA and Nogales, Sonora, Mexico as a case study. I ask the following questions: What is the spatial distribution of vulnerability, and what is the role of the border in increasing or decreasing vulnerability? What kind of coordination should occur among local institutions to address flooding in the cities? I use a Geographic Information System to analyze the spatial distribution of flood events and the socio-economic characteristics of both cities. The result is an index that estimates flood vulnerability using a set of indicators that are comparable between cities on both sides of the border. I interviewed planners and local government officials to validate the vulnerability model and to assess collaboration efforts between the cities. This research contributes to our understanding of vulnerability and sustainability in two ways: (1) it provides a framework for assessing and comparing vulnerabilities at the city level between nations, overcoming issues of data incompatibility, and (2) it highlights the institutional arrangements of border cities and how they affect vulnerability.