【正文】 ntal impacts assessment is necessary while for feasibility study of projects the environmental impact statement should be prepared. purposes of the assessment The purpose of EIA is to assess the environmental effects due to river basin development playing or proposed hydroelectric project .For the purpose of rationally utilizing natural resources, protecting the environment, improving environmental quality, and maintaining the ecological balance, the optimum plan can be screened out through the parison of the technical, economical and environmental indices of the alternative plans of the project. Besides, the corresponding mitigation measures for the adverse effects and the improvement measures for the beneficial effects should be put forwards during various stages, such as planning, design, construction, and management. The work of EIA is very important, as EIA (s) is the fundamental document for decision making and policy arrangement for the project. The development of EIA makes it possible to change the work of environmental protection from a status of passive control into a status of active prevention In addition, the most important point is that through the work of EIA the project could develop more prehensive benefits and eliminate the adverse effect. The classification of the assessmentAccording to the temporal and spatial dimensions the environmental impact assessment can be classified into two categories. From temporal dimension it can be further classified as the retrospective environmental impact assessment for exiting projects, the present environmental impact assessment for project under construction and the prospective environmental impact assessment for projects under planning. Generally speaking, the environmental impact assessment refers almost all to the prospective EIA. From spatial dimension it can be classified as assessment for individual project, for a system of projects, and even for all the projects included in the river basin planning. The depth of work for environmental assessment should be patible with stage of planning and design. In the river basin planning stage, the environmental assessment should be made for the whole basin, and a preliminary suggestion for mitigation measures of the adverse effects should be proposed. If necessary, reports on special topics should be provided for significant impacts. In the feasibility study stage, the environmental assessment for each of important parameters and prehensive chapter of environmental protection should be provided o give a detailed description for demonstration the environmental effect of project and implementing the mitigation and improvement measures for the adverse effects,. In technical design stage, an additional study should be made for the remaining key problems. In the stage of construction, the environmental prot6ection planning and the practicing schedule for the construction area and the reservoir region should be included. Methods and proceduresIn practice, methods are closely interconnected with procedures. According to the process of EIA. The methods used can be divided into two categories. One is for assessing the environmental change and impact of each individual parameter, and the other is for assessing the impact of the whole assessment, appropriate mitigation measures can be established, and prehensive indices and indicators for the whole project can be derived so as to facilitate the parison of alternative project designs. The assessment procedures consist of five main steps: impact identification, impact prediction, impact evaluation, mitigation and protective measures, and monitoring programs. Among the five steps the impact identification, impact prediction and impact evaluation are most important. For each step there are different methods and considerations. impact identificationThe steps taken to identify environmental parameters likely to have impacts are as follows:1 Understanding the characteristics of the project, such as backwater curve, change of hydraulic and hydrological regime (such as change of discharge and silt distribution).2 Selection of an existing similar project and carrying out retrospective environmental assessment for reference.3 Investigation and description of the status of the existing environmental setting and base line.4 Use of checklists of interaction matrices for impact identification.5 Proposing the parameters with likely impacts or the unknown parameters for further impact prediction.The purposes of this are to identify the significant environmental modification, and to estimate the probability that the impact will occur. Impact prediction begins with quality identification, then simple methods are used for quantification and finally multifactor modeling is used for detailed quantification. Some of the methods might be classified as follows:1 Mathematical modeling of empirical formula (such as thereservoir and so on).2 Investigation and measurement (such as throughinvestigation of the scope of distribution of terrestrial flora and fauna within the inundated zone to predict the impact on them, the same method is used for prediction of the impact on historic and archaeological sites).3 Analysis of the effects of changes in the hydraulic and hydrological regime (such as through the study of change of flow and silt patterns to predict the areas influenced or affected by flood, waterlogging and salinity downstream, or through the change of habitats of flora and fauna to predict the future condition of the different species).4 Analogy or parison with existing projects (such as the use of parison to identify the change in water temperature qualitatively). Impact evaluation1. Environmental impact of each individual environmental parameter. One must investigate the change in environmental quality, propose the remedial measures for adverse effects, calculate the relationship between benefits and costs, and see whether t