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DRIFT (Downstream Response to Imposed Flow Transformations) is a scenario-based environmental flow assessment methodology applied during impact studies associated with the Lesotho Highlands Water Project, Southern Africa. DRIFT offers a structured process for predicting the biophysical, social and economic consequences of altering a river's flow regime. The fish component of DRIFT provides a ten-step protocol designed to make such predictions using field data on a river's fish fauna linked to information on flow-related aspects of fish biology drawn from the literature and the knowledge base and professional experience of fish ecologists. A worked example of the methodology is presented based on a study site downstream from Katse Dam on the Malibamatso River, Lesotho, where the ecological consequences of four flow scenarios were evaluated. DRIFT and its fish component have emerged from studies in a semi-arid, developing region where unpredictable hydrological regimes and data scarcity constrain the prediction of ecological responses to flow regulation. Faced with similar information constraints, scientific uncertainty and limited windows of opportunity to guide water management, other countries have employed scientific panels to recommend environmental flows. DRIFT and its fish component compare favourably with recommended best practice for Australian scientific panel assessments of the flow requirements of river ecosystems. The risks associated with use of scientific panel approaches are discussed and minimum data sets and standards are recommended for the conduct of a DRIFT fish assessment. DRIFT and related frameworks represent the second level in a three-tiered hierarchy of environmental flow methods. They can provide environmental flow recommendations of far greater scientific resolution than hydrological methods by integrating many types of information on the responses of riverine biota to flow modifications. However, DRIFT should only be applied within an adaptive management framework where there is a genuine commitment to the generation and use of new knowledge derived from monitoring and research. Copyright (C) 2003 John Wiley Sons, Ltd.